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  • Bald Eagle

    The bald eagle (Haliaeetus leucocephalus) is a bird of prey found in North America. A sea eagle, it has two known subspecies and forms a species pair with the white-tailed eagle (Haliaeetus albicilla), which occupies the same niche as the bald eagle in the Palearctic. Its range includes most of Canada and Alaska, all of the contiguous United States, and northern Mexico. It is found near large bodies of open water with an abundant food supply and old-growth trees for nesting.

    The bald eagle is an opportunistic feeder which subsists mainly on fish, which it swoops down upon and snatches from the water with its talons. It builds the largest nest of any North American bird and the largest tree nests ever recorded for any animal species, up to 4 m (13 ft) deep, 2.5 m (8.2 ft) wide, and 1 metric ton (1.1 short tons) in weight. Sexual maturity is attained at the age of four to five years.

    Bald eagles are not bald; the name derives from an older meaning of the word, “white-headed”. The adult is mainly brown with a white head and tail. The sexes are identical in plumage, but females are about 25 percent larger than males. The yellow beak is large and hooked. The plumage of the immature is brown.

    The bald eagle is the national symbol of the United States and appears on its seal. In the late 20th century it was on the brink of extirpation in the contiguous United States, but measures such as banning the practice of hunting bald eagles and banning the use of the harmful pesticide DDT slowed the decline of their population. Populations have since recovered, and the species’ status was upgraded from “endangered” to “threatened” in 1995 and removed from the list altogether in 2007. In 2024, the bald eagle was officially made the national bird of the United States.[4]

    Taxonomy

    Closeup of the eponymous white head

    The bald eagle is placed in the genus Haliaeetus (sea eagles), and gets both its common and specific scientific names from the distinctive appearance of the adult’s head. Bald in the English name is from an older usage meaning “having white on the face or head” rather than “hairless”, referring to the white head feathers contrasting with the darker body.[5]

    The genus name is Neo-LatinHaliaeetus (from the Ancient Greek: ἁλιάετος, romanizedhaliaetoslit.‘sea eagle’),[6] and the specific name, leucocephalus, is Latinized (Ancient Greek: λευκός, romanizedleukoslit.‘white’)[7] and (κεφαλή, kephalḗ, ‘head’).[8][9]

    The bald eagle was one of the many species originally described by Carl Linnaeus in his 18th-century work Systema Naturae, under the name Falco leucocephalus.[10]

    The bald eagle forms a species pair with the white-tailed eagle of Eurasia. This species pair consists of a white-headed and a tan-headed species of roughly equal size; the white-tailed eagle also has overall somewhat paler brown body plumage. The two species fill the same ecological niche in their respective ranges. The pair diverged from other sea eagles at the beginning of the Early Miocene (c. 10 Ma BP) at the latest, but possibly as early as the Early/Middle Oligocene, 28 Ma BP, if the most ancient fossil record is correctly assigned to this genus.[11]

    Subspecies

    There are two recognized subspecies of bald eagle:[12][13]

    • H. l. leucocephalus (Linnaeus, 1766) is the nominate subspecies. It is found in the southern United States and Baja California Peninsula.[14]
    • H. l. washingtoniensis (Audubon, 1827), synonym H. l. alascanus (Townsend, 1897), the northern subspecies, is larger than southern nominate leucocephalus. It is found in the northern United States, Canada and Alaska.[12][14]

    Description

    A portrait style photo of a bald eagle emphasizing its feathers.
    Bald eagle plumage

    The plumage of an adult bald eagle is evenly dark brown with a white head and tail. The tail is moderately long and slightly wedge-shaped. Males and females are identical in plumage coloration, but sexual dimorphism is evident in the species, in that females are 25% larger than males.[12] The beak, feet and irises are bright yellow. The legs are feather-free, and the toes are short and powerful with large talons. The highly developed talon of the hind toe is used to pierce the vital areas of prey while it is held immobile by the front toes.[15] The beak is large and hooked, with a yellow cere.[16] The adult bald eagle is unmistakable in its native range. The closely related African fish eagle (Haliaeetus vocifer) (from far outside the bald eagle’s range) also has a brown body (albeit of somewhat more rufous hue), white head and tail, but differs from the bald eagle in having a white chest and black tip to the bill.[17]

    Bald eagle anatomy

    The plumage of the immature is a dark brown overlaid with messy white streaking until the fifth (rarely fourth, very rarely third) year, when it reaches sexual maturity.[12][15] Immature bald eagles are distinguishable from the golden eagle (Aquila chrysaetos), the only other very large, non-vulturine raptorial bird in North America, in that the former has a larger, more protruding head with a larger beak, straighter edged wings which are held flat (not slightly raised) and with a stiffer wing beat and feathers which do not completely cover the legs. When seen well, the golden eagle is distinctive in plumage with a more solid warm brown color than an immature bald eagle, with a reddish-golden patch to its nape and (in immature birds) a highly contrasting set of white squares on the wing.[18]

    A bald eagle showing its wingspan
    Closeup of a museum specimen’s foot, showing the toepads’ spiny papillae

    The bald eagle has sometimes been considered the largest true raptor (accipitrid) in North America. The only larger species of raptor-like bird is the California condor (Gymnogyps californianus), a New World vulture which today is not generally considered a taxonomic ally of true accipitrids.[19] However, the golden eagle, averaging 4.18 kg (9.2 lb) and 63 cm (25 in) in wing chord length in its American race (Aquila chrysaetos canadensis), is merely 455 g (1.003 lb) lighter in mean body mass and exceeds the bald eagle in mean wing chord length by around 3 cm (1.2 in).[17][20] Additionally, the bald eagle’s close cousins, the relatively longer-winged but shorter-tailed white-tailed eagle and the overall larger Steller’s sea eagle (Haliaeetus pelagicus), may, rarely, wander to coastal Alaska from Asia.[17]

    The bald eagle has a body length of 70–102 cm (28–40 in). Typical wingspan is between 1.8 and 2.3 m (5 ft 11 in and 7 ft 7 in) and mass is normally between 3 and 6.3 kg (6.6 and 13.9 lb).[17] Females are about 25% larger than males, averaging as much as 5.6 kg (12 lb), and against the males’ average weight of 4.1 kg (9.0 lb).[12][21][22][23]

    The size of the bird varies by location and generally corresponds with Bergmann’s rule: the species increases in size further away from the equator and the tropics. For example, eagles from South Carolina average 3.27 kg (7.2 lb) in mass and 1.88 m (6 ft 2 in) in wingspan, smaller than their northern counterparts.[24] One field guide in Florida listed similarly small sizes for bald eagles there, at about 4.13 kg (9.1 lb).[25] Of intermediate size, 117 migrant bald eagles in Glacier National Park were found to average 4.22 kg (9.3 lb) but this was mostly (possibly post-dispersal) juvenile eagles, with 6 adults here averaging 4.3 kg (9.5 lb).[26] Wintering eagles in Arizona (winter weights are usually the highest of the year since, like many raptors, they spend the highest percentage of time foraging during winter) were found to average 4.74 kg (10.4 lb).[27]

    The largest eagles are from Alaska, where large females may weigh more than 7 kg (15 lb) and span 2.44 m (8 ft 0 in) across the wings.[16][28] A survey of adult weights in Alaska showed that females there weighed on average 5.35 kg (11.8 lb), respectively, and males weighed 4.23 kg (9.3 lb) against immatures which averaged 5.09 kg (11.2 lb) and 4.05 kg (8.9 lb) in the two sexes.[29][30] An Alaskan adult female eagle that was considered outsized weighed some 7.4 kg (16 lb).[31] R.S. Palmer listed a record from 1876 in Wyoming County, New York of an enormous adult bald eagle that was shot and reportedly scaled 8.2 kg (18 lb).[30] Among standard linear measurements, the wing chord is 51.5–69 cm (20.3–27.2 in), the tail is 23–37 cm (9.1–14.6 in) long, and the tarsus is 8 to 11 cm (3.1 to 4.3 in).[17][32] The culmen reportedly ranges from 3 to 7.5 cm (1.2 to 3.0 in), while the measurement from the gape to the tip of the bill is 7–9 cm (2.8–3.5 in).[32][33] The bill size is unusually variable: Alaskan eagles can have up to twice the bill length of birds from the southern United States (GeorgiaLouisiana, Florida), with means including both sexes of 6.83 cm (2.69 in) and 4.12 cm (1.62 in) in culmen length, respectively, from these two areas.[34][35]

    The call consists of weak staccato, chirping whistles, kleek kik ik ik ik, somewhat similar in cadence to a gull‘s call. The calls of young birds tend to be more harsh and shrill than those of adults.[17][18]

    Range

    Bald eagle in flight at Yellowstone National Park, Wyoming

    The bald eagle’s natural range covers most of North America, including most of Canada, all of the continental United States, and northern Mexico. It is the only sea eagle endemic to North America. Occupying varied habitats from the bayous of Louisiana to the Sonoran Desert and the eastern deciduous forests of Quebec and New England, northern birds are migratory, while southern birds are resident, remaining on their breeding territory all year. At minimum population, in the 1950s, it was largely restricted to Alaska, the Aleutian Islands, northern and eastern Canada, and Florida.[36] From 1966 to 2015 bald eagle numbers increased substantially throughout its winter and breeding ranges,[37] and as of 2018 the species nests in every continental state and province in the United States and Canada.[38]

    The majority of bald eagles in Canada are found along the British Columbia coast while large populations are found in the forests of Alberta, Saskatchewan, Manitoba and Ontario.[39] Bald eagles also congregate in certain locations in winter. From November until February, one to two thousand birds winter in Squamish, British Columbia, about halfway between Vancouver and Whistler. In March 2024, bald eagles were found nesting in Toronto for the first time.[40] The birds primarily gather along the Squamish and Cheakamus Rivers, attracted by the salmon spawning in the area.[41] Similar congregations of wintering bald eagles at open lakes and rivers, wherein fish are readily available for hunting or scavenging, are observed in the northern United States.[42]

    It has occurred as a vagrant twice in Ireland; a juvenile was shot illegally in Fermanagh on January 11, 1973 (misidentified at first as a white-tailed eagle), and an exhausted juvenile was captured near CastleislandCounty Kerry on November 15, 1987.[43] There is also a record of it from Llyn Coron, Anglesey, in the United Kingdom, from October 17, 1978;[44] the provenance of this individual eagle has remained in dispute.

    Habitat

    In flight during a licensed performance in Ontario, Canada
    During training at the Canadian Raptor Conservancy

    The bald eagle occurs during its breeding season in virtually any kind of American wetland habitat such as seacoasts, rivers, large lakes or marshes or other large bodies of open water with an abundance of fish. Studies have shown a preference for bodies of water with a circumference greater than 11 km (7 mi), and lakes with an area greater than 10 km2 (4 sq mi) are optimal for breeding bald eagles.[45]

    The bald eagle typically requires old-growth and mature stands of coniferous or hardwood trees for perching, roosting, and nesting. Tree species reportedly is less important to the eagle pair than the tree’s height, composition and location.[46] Perhaps of paramount importance for this species is an abundance of comparatively large trees surrounding the body of water. Selected trees must have good visibility, be over 20 m (66 ft) tall, an open structure, and proximity to prey. If nesting trees are in standing water such as in a mangrove swamp, the nest can be located fairly low, at as low as 6 m (20 ft) above the ground.[47] In a more typical tree standing on dry ground, nests may be located from 16 to 38 m (52 to 125 ft) in height. In Chesapeake Bay, nesting trees averaged 82 cm (32 in) in diameter and 28 m (92 ft) in total height, while in Florida, the average nesting tree stands 23 m (75 ft) high and is 23 cm (9.1 in) in diameter.[48][49] Trees used for nesting in the Greater Yellowstone area average 27 m (89 ft) high.[50] Trees or forest used for nesting should have a canopy cover of no more than 60%, and no less than 20%, and be in close proximity to water.[45] Most nests have been found within 200 m (660 ft) of open water. The greatest distance from open water recorded for a bald eagle nest was over 3 km (1.9 mi), in Florida.[19]

    Bald eagle nests are often very large in order to compensate for size of the birds. The largest recorded nest was found in Florida in 1963, and was measured at 2.9 m (9.5 ft) wide and 6.1 m (20 ft) deep.[51]

    In Florida, nesting habitats often consist of mangrove swamps, the shorelines of lakes and rivers, pinelands, seasonally flooded flatwoodshardwood swamps, and open prairies and pastureland with scattered tall trees. Favored nesting trees in Florida are slash pines (Pinus elliottii), longleaf pines (P. palustris), loblolly pines (P. taeda) and cypress trees, but for the southern coastal areas where mangroves are usually used.[47] In Wyoming, groves of mature cottonwoods or tall pines found along streams and rivers are typical bald eagle nesting habitats. Wyoming eagles may inhabit habitat types ranging from large, old-growth stands of ponderosa pines (Pinus ponderosa) to narrow strips of riparian trees surrounded by rangeland.[19] In Southeast AlaskaSitka spruce (Picea sitchensis) provided 78% of the nesting trees used by eagles, followed by hemlocks (Tsuga) at 20%.[46] Increasingly, eagles nest in human-made reservoirs stocked with fish.[47]

    With freshly caught fish in Kodiak

    The bald eagle is usually quite sensitive to human activity while nesting, and is found most commonly in areas with minimal human disturbance. It chooses sites more than 1.2 km (0.75 mi) from low-density human disturbance and more than 1.8 km (1.1 mi) from medium- to high-density human disturbance.[45] However, bald eagles will occasionally nest in large estuaries or secluded groves within major cities, such as Hardtack Island on the Willamette River in Portland, Oregon or John Heinz National Wildlife Refuge at Tinicum in Philadelphia, Pennsylvania, which are surrounded by a great quantity of human activity.[52][53] Even more contrary to the usual sensitivity to disturbance, a family of bald eagles moved to the Harlem neighborhood in New York City in 2010.[54]

    While wintering, bald eagles tend to be less habitat and disturbance sensitive. They will commonly congregate at spots with plentiful perches and waters with plentiful prey and (in northern climes) partially unfrozen waters. Alternately, non-breeding or wintering bald eagles, particularly in areas with a lack of human disturbance, spend their time in various upland, terrestrial habitats sometimes quite far away from waterways. In the northern half of North America (especially the interior portion), this terrestrial inhabitance by bald eagles tends to be especially prevalent because unfrozen water may not be accessible. Upland wintering habitats often consist of open habitats with concentrations of medium-sized mammals, such as prairiesmeadows or tundra, or open forests with regular carrion access.[19][46]

    Behavior

    The bald eagle is a powerful flier, and soars on thermal convection currents. It reaches speeds of 56–70 km/h (35–43 mph) when gliding and flapping, and about 48 km/h (30 mph) while carrying fish.[55] Its dive speed is between 120–160 km/h (75–99 mph), though it seldom dives vertically.[56] Regarding their flying abilities, despite being morphologically less well adapted to faster flight than golden eagles (especially during dives), the bald eagle is considered surprisingly maneuverable in flight. Bald eagles have also been recorded catching up to and then swooping under geese in flight, turning over and thrusting their talons into the other bird’s breast.[30] It is partially migratory, depending on location. If its territory has access to open water, it remains there year-round, but if the body of water freezes during the winter, making it impossible to obtain food, it migrates to the south or to the coast. A number of populations are subject to post-breeding dispersal, mainly in juveniles; Florida eagles, for example, will disperse northwards in the summer.[57] The bald eagle selects migration routes which take advantage of thermalsupdrafts, and food resources. During migration, it may ascend in a thermal and then glide down, or may ascend in updrafts created by the wind against a cliff or other terrain. Migration generally takes place during the daytime, usually between the local hours of 8:00 a.m. and 6:00 p.m., when thermals are produced by the sun.[15]

    Diet and feeding

    The bald eagle is an opportunistic carnivore with the capacity to consume a great variety of prey. Fish often comprise most of the eagle’s diet throughout their range.[58] In 20 food habit studies across the species’ range, fish comprised 56% of the diet of nesting eagles, birds 28%, mammals 14% and other prey 2%.[59] More than 400 species are known to be included in the bald eagle’s prey spectrum, far more than its ecological equivalent in the Old World, the white-tailed eagle, is known to take. Despite its considerably lower population, the bald eagle may come in second amongst all North American accipitrids, slightly behind only the red-tailed hawk, in number of prey species recorded.[30][59][60][61]

    Behavior

    Juvenile with salmonKatmai National Park

    To hunt fish, the eagle swoops down over the water and snatches the fish out of the water with its talons. They eat by holding the fish in one claw and tearing the flesh with the other. Eagles have structures on their toes called spicules that allow them to grasp fish. Ospreys also have this adaptation.[55] Bird prey may occasionally be attacked in flight, with prey up to the size of Canada geese attacked and killed in mid-air.[62] It has been estimated that the bald eagle’s gripping power (pounds by square inch) is ten times greater than that of a human.[63] Bald eagles can fly with fish at least equal to their own weight, but if the fish is too heavy to lift, the eagle may be dragged into the water. Bald eagles can swim, but in some cases, they drag their catch ashore with their talons. Still, some eagles drown or succumb to hypothermia.[64] Many sources claim that bald eagles, like all large eagles, cannot normally take flight carrying prey more than half of their own weight unless aided by favorable wind conditions.[47][65] On numerous occasions, when large prey such as large fish including mature salmon or geese are attacked, eagles have been seen to make contact and then drag the prey in a strenuously labored, low flight over the water to a bank, where they then finish off and dismember the prey.[32][30][59][60] When food is abundant, an eagle can gorge itself by storing up to 1 kg (2.2 lb) of food in a pouch in the throat called a crop. Gorging allows the bird to fast for several days if food becomes unavailable.[47] Occasionally, bald eagles may hunt cooperatively when confronting prey, especially relatively large prey such as jackrabbits or herons, with one bird distracting potential prey, while the other comes behind it in order to ambush it.[16][66][67] While hunting waterfowl, bald eagles repeatedly fly at a target and cause it to dive repeatedly, hoping to exhaust the victim so it can be caught (white-tailed eagles have been recorded hunting waterfowl in the same way). When hunting concentrated prey, a successful catch often results in the hunting eagle being pursued by other eagles and needing to find an isolated perch for consumption if it is able to carry it away successfully.[32]

    They obtain much of their food as carrion or via a practice known as kleptoparasitism, by which they steal prey away from other predators. Due to their dietary habits, bald eagles are frequently viewed in a negative light by humans.[19] Thanks to their superior foraging ability and experience, adults are generally more likely to hunt live prey than immature eagles, which often obtain their food from scavenging.[68][69] They are not very selective about the condition or origin, whether provided by humans, other animals, auto accidents or natural causes, of a carcass’s presence, but will avoid eating carrion where disturbances from humans are a regular occurrence. They will scavenge carcasses up to the size of whales, though carcasses of ungulates and large fish are seemingly preferred.[32] Congregated wintering waterfowl are frequently exploited for carcasses to scavenge by immature eagles in harsh winter weather.[70] Bald eagles also may sometimes feed on material scavenged or stolen from campsites and picnics, as well as garbage dumps (dump usage is habitual mainly in Alaska)[71] and fish-processing plants.[72]

    Fish

    In flight with freshly caught fish
    Feeding on catfish and other various fishes.[73] Painted by John James Audubon

    In Southeast Alaska, fish comprise approximately 66% of the year-round diet of bald eagles and 78% of the prey brought to the nest by the parents.[74] Eagles living in the Columbia River Estuary in Oregon were found to rely on fish for 90% of their dietary intake.[75] At least 100 species of fish have been recorded in the bald eagle’s diet.[60] From observation in the Columbia River, 58% of the fish were caught alive by the eagle, 24% were scavenged as carcasses and 18% were pirated away from other animals.[75]

    In the Pacific Northwest, spawning trout and salmon provide most of the bald eagles’ diet from late summer throughout fall.[76] Though bald eagles occasionally catch live salmon, they usually scavenge spawned salmon carcass.[77][78] Southeast Alaskan eagles largely prey on pink salmon (Oncorhynchus gorbuscha), coho salmon (O. kisutch) and, more locally, sockeye salmon (O. nerka), with Chinook salmon (O. tshawytscha).[74] Due to the Chinook salmon’s large size (12 to 18 kg (26 to 40 lb) average adult size) probably being taken only as carrion and a single carcass can attract several eagles.[74] Also important in the estuaries and shallow coastlines of southern Alaska are Pacific herring (Clupea pallasii), Pacific sand lance (Ammodytes hexapterus) and eulachon (Thaleichthys pacificus).[74] In Oregon’s Columbia River Estuary, the most significant prey species were largescale suckers (Catostomus macrocheilus) (17.3% of the prey selected there), American shad (Alosa sapidissima; 13%) and common carp (Cyprinus carpio; 10.8%).[75] Eagles living in the Chesapeake Bay in Maryland were found to subsist largely on American gizzard shad (Dorosoma cepedianum), threadfin shad (Dorosoma petenense) and white bass (Morone chrysops).[79] Floridian eagles have been reported to prey on catfish, most prevalently the brown bullhead (Ameiurus nebulosus) and any species in the genus Ictalurus as well as mullettroutneedlefish, and eels.[19][47][80] Chain pickerels (Esox niger) and white suckers (Catostomus commersonii) are frequently taken in interior Maine.[81] Wintering eagles on the Platte River in Nebraska preyed mainly on American gizzard shads and common carp.[82] Bald eagles are also known to eat the following fish species: rainbow trout (Oncorhynchus mykiss), white catfish (Ameiurus catus), rock greenling (Hexagrammos lagocephalus), Pacific cod (Gadus macrocephalus), Atka mackerel (Pleurogrammus monopterygius), largemouth bass (Micropterus salmoides), northern pike (Esox lucius), striped bass (Morone saxatilis), dogfish shark (Squalidae.sp) and Blue walleye (Sander vitreus).[83][84][85]

    Fish taken by bald eagles varies in size, but bald eagles take larger fish than other piscivorous birds in North America, typically range from 20 to 75 cm (7.9 to 29.5 in) and prefer 36 cm (14 in) fish.[86] When experimenters offered fish of different sizes in the breeding season around Lake Britton in California, fish measuring 34 to 38 cm (13 to 15 in) were taken 71.8% of the time by parent eagles while fish measuring 23 to 27.5 cm (9.1 to 10.8 in) were chosen only 25% of the time.[87] At nests around Lake Superior, the remains of fish (mostly suckers) were found to average 35.4 cm (13.9 in) in total length.[88] In the Columbia River estuary, most preyed on by eagles were estimated to measure less than 30 cm (12 in), but larger fish between 30 and 60 cm (12 and 24 in) or even exceeding 60 cm (24 in) in length also taken especially during the non-breeding seasons.[75] They can take fish up to at least twice their own weight, such as large mature salmonscarps, or even muskellunge (Esox masquinongy), by dragging its catch with talons and pull toward ashore.[30][59][89][90] Much larger marine fish such as Pacific halibut (Hippoglossus stenolepis) and lemon sharks (Negaprion brevirostris) have been recorded among bald eagle prey though probably are only taken as young, as small, newly mature fish, or as carrion.[61][91]

    Benthic fishes such as catfish are usually consumed after they die and float to the surface, though while temporarily swimming in the open may be more vulnerable to predation than most fish since their eyes focus downwards.[79] Bald eagles also regularly exploit water turbines which produce battered, stunned or dead fish easily consumed.[92] Predators who leave behind scraps of dead fish that they kill, such as brown bears (Ursus arctos), gray wolves (Canis lupus) and red foxes (Vulpes vulpes), may be habitually followed in order to scavenge the kills secondarily.[74] Once North Pacific salmon die off after spawning, usually local bald eagles eat salmon carcasses almost exclusively. Eagles in Washington need to consume 489 g (1.078 lb) of fish each day for survival, with adults generally consuming more than juveniles and thus reducing potential energy deficiency and increasing survival during winter.[93]

    Birds

    Bald eagle attacking an American coot

    Behind fish, the next most significant prey base for bald eagles are other waterbirds. The contribution of such birds to the eagle’s diet is variable, depending on the quantity and availability of fish near the water’s surface. Waterbirds can seasonally comprise from 7% to 80% of the prey selection for eagles in certain localities.[75][94] Overall, birds are the most diverse group in the bald eagle’s prey spectrum, with 200 prey species recorded.[30][60][61]

    Bird species most preferred as prey by eagles tend to be medium-sized, such as western grebes (Aechmophorus occidentalis), mallards (Anas platyrhynchos), and American coots (Fulica americana) as such prey is relatively easy for the much larger eagles to catch and fly with.[19][75] American herring gull (Larus smithsonianus) are the favored avian prey species for eagles living around Lake Superior.[88] Black ducks (Anas rubripes), common eiders (Somateria mollissima), and double-crested cormorants (Phalacrocorax auritus) are also frequently taken in coastal Maine[81] and velvet scoter (Melanitta fusca) was dominant prey in San Miguel Island.[95]

    A bald eagle prepares to pick off a common murre from Colony Rock in Oregon Islands National Wildlife Refuge, Oregon, United States.

    Due to easy accessibility and lack of formidable nest defense against eagles by such species, bald eagles are capable of preying on such seabirds at all ages, from eggs to mature adults, and they can effectively cull large portions of a colony.[96] Along some portions of the North Pacific coastline, bald eagles which had historically preyed mainly kelp-dwelling fish and supplementally sea otter (Enhydra lutris) pups are now preying mainly on seabird colonies since both the fish (possibly due to overfishing) and otters (cause unknown) have had steep population declines, causing concern for seabird conservation.[97] Because of this more extensive predation, some biologist has expressed concern that murres are heading for a “conservation collision” due to heavy eagle predation.[96] Eagles have been confirmed to attack nocturnally active, burrow-nesting seabird species such as storm petrels and shearwaters by digging out their burrows and feeding on all animals they find inside.[98] If a bald eagle flies close by, waterbirds will often fly away en masse, though they may seemingly ignore a perched eagle in other cases. when the birds fly away from a colony, this exposes their unprotected eggs and nestlings to scavengers such as gulls.[96]

    While they usually target small to medium-sized seabirds, larger seabirds such as great black-backed gulls (Larus marinus) and northern gannets (Morus bassanus) and brown pelicans (Pelecanus occidentalis) of all ages can successfully be taken by bald eagles.[99][100][101] Similarly, large waterbirds are occasionally killed. Geese such as wintering emperor geese (Chen canagica) and snow geese (C. caerulescens), which gather in large groups, sometimes becoming regular prey.[32][65] Smaller Ross’s geese (Anser rossii) are also taken, as well as large-sized Canada geese (Branta canadensis).[102][79] Predation on the largest subspecies (Branta canadensis maxima) has been reported.[103] Other large waterbird prey include common loons (Gavia immer) of all ages.[104] Large wading birds can also fall prey to bald eagles. For the great blue herons (Ardea herodias), bald eagles are their only serious enemies of all ages.[59][105] Slightly larger Sandhill cranes (Grus canadensis) can be taken as well.[106] While adult whooping cranes (Grus americana) are too large and formidable, their chicks can fall prey to bald eagles.[107][108][109] They even occasionally prey on adult tundra swans (Cygnus columbianus).[110] Young trumpeter swans (Cygnus buccinator) are also taken, and an unsuccessful attack on an adult swan has been photographed.[111][112]

    Bald eagles have been occasionally recorded as killing other raptors. In some cases, these may be attacks of competition or kleptoparasitism on rival species but end with the consumption of the dead victims. Nine species of other accipitrids and owls are known to have been preyed upon by bald eagles. Owl prey species have ranged in size from western screech-owls (Megascops kennicotti) to snowy owls (Bubo scandiacus).[30][60][61][113] Larger diurnal raptors known to have fallen victim to bald eagles have included red-tailed hawks (Buteo jamaicensis),[114] peregrine falcons (Falco peregrinus),[115] northern goshawks (Accipiter gentilis),[116] ospreys (Pandion haliaetus)[117] and black (Coragyps atratus) and turkey vultures (Cathartes aura).[118]

    Mammals

    Carrying a caught cottontail rabbit in Seedskadee National Wildlife Refuge
    A bald eagle on a whale carcass

    Mammalian preys are generally less frequently taken than fish or avian prey. However, in some regions, such as landlocked areas of North America, wintering bald eagles may become habitual predators of medium-sized mammals that occur in colonies or local concentrations, such as prairie dogs (Cynomys sp.) and jackrabbits (Lepus sp.).[19][119] Bald eagles in Seedskadee National Wildlife Refuge often hunt in pair to catch cottontails, jackrabbits and prairie dogs.[120] They can attack and prey on rabbits and hares of nearly any size, from marsh rabbits (Sylvilagus palustris) to black and white-tailed jackrabbits (Lepus californicus & L. townsendii), and Arctic hares (Lepus arcticus).[121][122][85] In San Luis Valley, white-tailed jackrabbits can be important prey.[110] Additionally, rodents such as montane voles (Microtus montanus), brown rats (Rattus norvegicus), and various squirrels are taken as supplementary prey.[122][15][81] Larger rodents such as muskrats (Ondatra zibethicus), young or small adult nutrias (Myocastor coypus) and groundhogs (Marmota monax) are also preyed upon.[123][124] Even American porcupines (Erethizon dorsatum) are reportedly attacked and killed.[125]

    Where available, seal colonies can provide a lot of food. On Protection IslandWashington, they commonly feed on harbor seal (Phoca vitulina) afterbirths, still-borns and sickly seal pups.[126] Similarly, bald eagles in Alaska readily prey on sea otter (Enhydra lutris) pups.[127] Small to medium-sized terrestrial mammalian carnivores can be taken infrequently. Mustelid including American martens (Martes pennanti),[128] American minks (Neogale vison),[129] and larger fisher cats (Pekania pennanti) are known to be hunted.[130] Foxes are also taken, including Island foxes ( Urocyon littoralis ),[131] Arctic foxes (Vulpes lagopus),[132] and grey foxes (Urocyon cinereoargenteus).[133] Although fox farmers claimed that bald eagle heavily prey on young and adult free-range Arctic fox, the predation events are sporadic.[134][135] In one instance, two bald Eagles fed upon a red fox (Vulpes vulpes) that had tried to cross a frozen Delaware Lake.[136] Other medium-sized carnivorans such as striped skunks (Mephitis mephitis),[137] American hog-nosed skunks (Conepatus leuconotus),[138] and common raccoons (Procyon lotor)[133] are taken, as well as domestic cats (Felis catus) and dogs (canis familiaris).[139][134]

    Other wild mammalian prey include fawns of deer such as white-tailed deer (Odocoileus virginianus) and Sitka deer (Odocoileus hemionus sitkensis), which weigh around 3 kg (6.6 lb) can be taken alive by bald eagles.[140][141] In one instance, a bald eagle was observed carrying 6.8 kg (15 lb) mule deer (Odocoileus hemionus) fawn.[142] Additionally, Virginia opossums (Didelphis virginiana) can be preyed upon. Still, predation events are rare due to their nocturnal habits.[121][143][133]

    Together with the golden eagle, bald eagles are occasionally accused of preying on livestock, especially sheep (Ovis aries). There are a handful of proven cases of lamb predation, some specimens weighing up to 11 kg (24 lb), by bald eagles. Still, they are much less likely to attack a healthy lamb than a golden eagle. Both species prefer native, wild prey and are unlikely to cause any extensive detriment to human livelihoods.[144] There is one case of a bald eagle killing and feeding on an adult, pregnant ewe (then joined in eating the kill by at least 3 other eagles), which, weighing on average over 60 kg (130 lb), is much larger than any other known prey taken by this species.[145]

    Reptiles and other prey

    Supplemental prey is readily taken given the opportunity. In some areas, reptiles may become regular prey, especially in warm areas such as Florida where reptile diversity is high. Turtles are perhaps the most regularly hunted type of reptile.[19] In coastal New Jersey, 14 of 20 studied eagle nests included remains of turtles. The main species found were common musk turtles (Sternotherus odoratus), diamondback terrapin (Malaclemys terrapin) and juvenile common snapping turtles (Chelydra serpentina). In these New Jersey nests, mainly subadult and small adults were taken, ranging in carapace length from 9.2 to 17.1 cm (3.6 to 6.7 in).[146] Similarly, many turtles were recorded in the diet in the Chesapeake Bay.[147] In Texassoftshell turtles are the most frequently taken prey,[148] and a large number of Barbour’s map turtles are taken in Torreya State Park.[149] Other reptilian and amphibian prey includes southern alligator lizards (Elgaria multicarinata),[95] snakes such as garter snakes and rattlesnakes,[47][150][151][85] and Greater siren (Siren lacertina).[121]

    Invertebrates are occasionally taken. In Alaska, eagles feed on sea urchins (Strongylocentrotus sp.), chitons, mussels, and crabs.[152] Other various mollusks such as land snailsabalonesbivalvesperiwinklesblue musselssquids, and starfishes are taken as well.[95]

    Interspecific predatory relationships

    Pursuing an osprey to steal fish

    When competing for food, eagles will usually dominate other fish-eaters and scavengers, aggressively displacing mammals such as coyotes (Canis latrans) and foxes, and birds such as corvidsgulls, vultures and other raptors.[71] Occasionally, coyotes, bobcats (Lynx rufus) and domestic dogs (Canis familiaris) can displace eagles from carrion, usually less confident immature birds, as has been recorded in Maine.[153] Bald eagles are less active, bold predators than golden eagles and get relatively more of their food as carrion and from kleptoparasitism (although it is now generally thought that golden eagles eat more carrion than was previously assumed).[95][20] However, the two species are roughly equal in size, aggressiveness and physical strength and so competitions can go either way. Neither species is known to be dominant, and the outcome depends on the size and disposition of the individual eagles involved.[32] Wintering bald and golden eagles in Utah both sometimes won conflicts, though in one recorded instance a single bald eagle successfully displaced two consecutive golden eagles from a kill.[154]

    Though bald eagles face few natural threats, an unusual attacker comes in the form of the common loon (G. immer), which is also taken by eagles as prey. While common loons normally avoid conflict, they are highly territorial and will attack predators and competitors by stabbing at them with their knife-like bill; as the range of the bald eagle has increased following conservation efforts, these interactions have been observed on several occasions, including a fatality of a bald eagle in Maine that is presumed to have come about as a result of it attacking a nest, then having a fatal puncture wound inflicted by one or both loon parents.[155]

    The bald eagle is thought to be much more numerous in North America than the golden eagle, with the bald species estimated to number at least 150,000 individuals, about twice as many as there are golden eagles estimated to live in North America.[20][38] Due to this, bald eagles often outnumber golden eagles at attractive food sources.[20] Despite the potential for contention between these animals, in New Jersey during winter, a golden eagle and numerous bald eagles were observed to hunt snow geese alongside each other without conflict. Similarly, both eagle species have been recorded, via video-monitoring, to feed on gut piles and carcasses of white-tailed deer (Odocoileus virginianus) in remote forest clearings in the eastern Appalachian Mountains without apparent conflict.[20] Bald eagles are frequently mobbed by smaller raptors, due to their infrequent but unpredictable tendency to hunt other birds of prey.[154] Many bald eagles are habitual kleptoparasites, especially in winters when fish are harder to come by. They have been recorded stealing fish from other predators such as ospreysherons and even otters.[32][156] They have also been recorded opportunistically pirating birds from peregrine falcons (Falco peregrinus), prairie dogs from ferruginous hawks (Buteo regalis) and even jackrabbits from golden eagles.[157][158] When they approach scavengers such as dogs, gulls or vultures at carrion sites, they often attack them in an attempt to force them to disgorge their food.[47] Healthy adult bald eagles are not preyed upon in the wild and are thus considered apex predators.[159]

    Reproduction

    Bald eagles are sexually mature at four or five years of age. When they are old enough to breed, they often return to the area where they were born. Bald eagles have high mate fidelity and generally mate for life. However, if one pair member dies or disappears, the survivor will choose a new mate. A pair that has repeatedly failed in breeding attempts may split and look for new mates.[160] Bald eagle courtship involves elaborate, spectacular calls and flight displays by the males. The flight includes swoops, chases, and cartwheels, in which they fly high, lock talons, and free-fall, separating just before hitting the ground.[59][161][162] Usually, a territory defended by a mature pair will be 1 to 2 km (0.62 to 1.24 mi) of waterside habitat.[19]

    Mating

    Compared to most other raptors, which mostly nest in April or May, bald eagles are early breeders: nest building or reinforcing is often by mid-February, egg laying is often late February (sometimes during deep snow in the North), and incubation is usually mid-March and early May. Eggs hatch from mid-April to early May, and the young fledge from late June to early July.[19] The nest is the largest of any bird in North America; it is used repeatedly over many years and with new material added each year may eventually be as large as 4 m (13 ft) deep, 2.5 m (8.2 ft) across and weigh 1 metric ton (1.1 short tons).[12] One nest in Florida was found to be 6.1 m (20 ft) deep, 2.9 meters (9.5 ft) across, and to weigh 3 short tons (2.7 metric tons).[163] This nest is on record as the largest tree nest ever recorded for any animal.[164] Usually nests are used for under five years, as they either collapse in storms or break the branches supporting them by their sheer weight. However, one nest in the Midwest was occupied continuously for at least 34 years.[47] The nest is built of branches, usually in large trees found near water. When breeding where there are no trees, the bald eagle will nest on the ground, as has been recorded largely in areas largely isolated from terrestrial predators, such as Amchitka Island in Alaska.[71]

    Egg, Collection at Museum Wiesbaden in Germany

    In Sonora, Mexico, eagles have been observed nesting on top of hecho catcuses (Pachycereus pectin-aboriginum).[165] Nests located on cliffs and rock pinnacles have been reported historically in California, KansasNevadaNew Mexico and Utah, but currently are only verified to occur only in Alaska and Arizona.[19] The eggs average about 73 mm (2.9 in) long, ranging from 58 to 85 mm (2.3 to 3.3 in), and have a breadth of 54 mm (2.1 in), ranging from 47 to 63 mm (1.9 to 2.5 in).[55][59] Eggs in Alaska averaged 130 g (4.6 oz) in mass, while in Saskatchewan they averaged 114.4 g (4.04 oz).[166][167] As with their ultimate body size, egg size tends to increase with distance from the equator.[59] Eagles produce between one and three eggs per year, two being typical. Rarely, four eggs have been found in nests, but these may be exceptional cases of polygyny.[134] Eagles in captivity have been capable of producing up to seven eggs.[168] It is rare for all three chicks to successfully reach the fledgling stage. The oldest chick often bears the advantage of a larger size and louder voice, which tends to draw the parents’ attention towards it.[19] Occasionally, as is recorded in many large raptorial birds, the oldest sibling sometimes attacks and kills its younger sibling(s), especially early in the nesting period when their sizes are most different.[19] However, nearly half of the known bald eagles produce two fledglings (more rarely three), unlike in some other “eagle” species such as some in the genus Aquila, in which a second fledgling is typically observed in less than 20% of nests, despite two eggs typically being laid.[29] Both the male and female take turns incubating the eggs, but the female does most of the sitting. The parent not incubating will hunt for food or look for nesting material during this stage. For the first two to three weeks of the nestling period, at least one adult is at the nest almost 100% of the time. After five to six weeks, the attendance of parents usually drops off considerably (with the parents often perching in trees nearby).[19]

    Adult and chick
    Chick at Everglades National Park

    A young eaglet can gain up to 170 g (6.0 oz) a day, the fastest growth rate of any North American bird.[47] The young eaglets pick up and manipulate sticks, play tug of war with each other, practice holding things in their talons, and stretch and flap their wings. By eight weeks, the eaglets are strong enough to flap their wings, lift their feet off the nest platform, and rise in the air.[47] The young fledge at anywhere from 8 to 14 weeks of age, though will remain close to the nest and be attended to by their parents for a further 6 weeks. Juvenile eagles first start dispersing away from their parents about 8 weeks after they fledge. Variability in departure date related to effects of sex and hatching order on growth and development.[167] For the next four years, immature eagles wander widely in search of food until they attain adult plumage and are eligible to reproduce.[169]

    Male eagles have been observed killing and cannibalizing their chicks.[170] In 2024 at the National Conservation Training Center in West Virginia, the NCTC’s Eagle Cam recorded two bald eagle chicks being attacked and devoured by their father as soon as the mother departed from the nest. The NCTC noted in its statement on the incident that such behavior “has been observed in other nests and is not uncommon in birds of prey.”[171]

    On rare occasions, bald eagles have been recorded to adopt other raptor fledglings into their nests, as seen in 2017 by a pair of eagles in Shoal Harbor Migratory Bird Sanctuary near Sidney, British Columbia. The pair of eagles in question are believed to have carried a juvenile red-tailed hawk back to their nest, presumably as prey, whereupon the chick was accepted into the family by both the parents and the eagles’ three nestlings.[172] The hawk, nicknamed “Spunky” by biologists monitoring the nest, fledged successfully.[173]

    Longevity and mortality

    Newly fledged juvenile

    The average lifespan of bald eagles in the wild is around 20 years, with the oldest confirmed one having been 38 years of age.[174] In captivity, they often live somewhat longer. In one instance, a captive individual in New York lived for nearly 50 years.[175] As with size, the average lifespan of an eagle population appears to be influenced by its location and access to prey.[176] As they are no longer heavily persecuted, adult mortality is quite low. In one study of Florida eagles, adult bald eagles reportedly had 100% annual survival rate.[20] In Prince William Sound in Alaska, adults had an annual survival rate of 88% even after the Exxon Valdez oil spill adversely affected eagles in the area.[177] Of 1,428 individuals from across the range necropsied by National Wildlife Health Center from 1963 to 1984, 329 (23%) eagles died from trauma, primarily impact with wires and vehicles; 309 (22%) died from gunshot; 158 (11%) died from poisoning; 130 (9%) died from electrocution; 68 (5%) died from trapping; 110 (8%) from emaciation; and 31 (2%) from disease; cause of death was undetermined in 293 (20%) of cases.[178] In this study, 68% of mortality was human-caused.[178] Today, eagle-shooting is believed to be considerably reduced due to the species’ protected status.[179] A U.S. Fish and Wildlife Service study of 1,490 bald eagle deaths from 1986 through 2017 in Michigan found that 532 (36%) died due to being struck by cars while scavenging roadkill and 176 (12%) died due to lead poisoning from ingesting fragments of lead ammo and fishing gear present in carrion, with the proportion of both causes of death increasing significantly towards the end of the study period.[180][181]

    Most non-human-related mortality involves nestlings or eggs. Around 50% of eagles survive their first year.[169] However, in the Chesapeake Bay area, 100% of 39 radio-tagged nestlings survived to their first year.[182] Nestling or egg fatalities may be due to nest collapses, starvation, sibling aggression or inclement weather. Another significant cause of egg and nestling mortality is predation. Nest predators include large gullscorvids (including ravens, crows and magpies), wolverines (Gulo gulo), fishers (Pekania pennanti), red-tailed hawks, owls, other eagles, bobcats, American black bears (Ursus americanus) and raccoons.[166][183][184][185][110][186][187][188] If food access is low, parental attendance at the nest may be lower because both parents may have to forage, thus resulting in less protection.[29] Nestlings are usually exempt from predation by terrestrial carnivores that are poor tree-climbers, but Arctic foxes (Vulpes lagopus) occasionally snatched nestlings from ground nests on Amchitka Island in Alaska before they were extirpated from the island.[71] The bald eagle will defend its nest fiercely from all comers and has even repelled attacks from bears, having been recorded knocking a black bear out of a tree when the latter tried to climb a tree holding nestlings.[189]

    Relationship with humans

    Population decline and recovery

    Inside a waste collection and transfer facility, in Homer, Alaska, United States

    Once a common sight in much of the continent, the bald eagle was severely affected in the mid-20th century by a variety of factors, among them the thinning of egg shells attributed to use of the pesticide DDT.[190] Bald eagles, like many birds of prey, were especially affected by DDT due to biomagnification. DDT itself was not lethal to the adult bird, but it interfered with their calcium metabolism, making them either sterile or unable to lay healthy eggs; many of their eggs were too brittle to withstand the weight of a brooding adult, making it nearly impossible for them to hatch.[36] It is estimated that in the early 18th century the bald eagle population was 300,000–500,000,[191] but by the 1950s there were only 412 nesting pairs in the 48 contiguous states of the US.[192][193] Other factors in bald eagle population reductions were a widespread loss of suitable habitat, as well as both legal and illegal shooting. In 1930 a New York City ornithologist wrote that in the territory of Alaska in the previous 12 years approximately 70,000 bald eagles had been shot. Many of the hunters killed the bald eagles under the long-held beliefs that bald eagles grabbed young lambs and even children with their talons, yet the birds were innocent of most of these alleged acts of predation (lamb predation is rare, human predation is thought to be non-existent).[194] Illegal shooting was described as “the leading cause of direct mortality in both adult and immature bald eagles” by the U.S. Fish and Wildlife Service in 1978.[195] Leading causes of death in bald eagles include lead pollution, poisoning, collision with motor vehicles, and power-line electrocution.[196] A study published in 2022 in the journal Science found that more than half of adult eagles across 38 US states suffered from lead poisoning.[197] The primary cause is when eagles scavenge carcasses of animals shot by hunters.[197] These are often tainted with lead shotgun pellets, rifle rounds, or fishing tackle.[197][181]

    The species was first protected in the U.S. and Canada by the 1918 Migratory Bird Treaty, later extended to all of North America. The Bald and Golden Eagle Protection Act, approved by the U.S. Congress in 1940, protected the bald eagle and the golden eagle, prohibiting commercial trapping and killing of the birds as well as collecting their eggs.[198] The bald eagle was declared an endangered species in the U.S. in 1967, and amendments to the 1940 act between 1962 and 1972 further restricted commercial uses and increased penalties for violators.[199][200] Perhaps most significant in the species’ recovery, in 1972, DDT was banned from usage in the United States due to the fact that it inhibited the reproduction of many birds.[201] DDT was completely banned in Canada in 1989, though its use had been highly restricted since the late 1970s.[202]

    First-year juvenile bald eagle at Anacortes, Washington, United States

    With regulations in place and DDT banned, the eagle population rebounded. The bald eagle can be found in growing concentrations throughout the United States and Canada, particularly near large bodies of water. In the early 1980s, the estimated total population was 100,000 individuals, with 110,000–115,000 by 1992;[12] the U.S. state with the largest resident population is Alaska, with about 40,000–50,000, with the next highest population the Canadian province of British Columbia with 20,000–30,000 in 1992.[12] Obtaining a precise count of the bald eagle population is extremely difficult. The most recent data submitted by individual states was in 2006, when 9789 breeding pairs were reported.[203] For some time, the stronghold breeding population of bald eagles in the lower 48 states was in Florida, where over a thousand pairs have held on while populations in other states were significantly reduced by DDT use. Today, the contiguous state with the largest number of breeding pairs of eagles is Minnesota with an estimated 1,312 pairs, surpassing Florida’s most recent count of 1,166 pairs. 23, or nearly half, of the 48 contiguous states now have at least 100 breeding pairs of bald eagles.[38] In Washington State, there were only 105 occupied nests in 1980. That number increased by about 30 per year, so that by 2005 there were 840 occupied nests. 2005 was the last year that the Washington Department of Fish and Wildlife counted occupied nests. Further population increases in Washington may be limited by the availability of late winter food, particularly salmon.[204]

    The bald eagle was officially removed from the U.S. federal government’s list of endangered species on July 12, 1995, by the U.S. Fish & Wildlife Service, when it was reclassified from “endangered” to “threatened”. On July 6, 1999, a proposal was initiated “To Remove the Bald Eagle in the Lower 48 States From the List of Endangered and Threatened Wildlife”. It was de-listed on June 28, 2007.[205] It has also been assigned a risk level of least concern category on the IUCN Red List.[2] In the Exxon Valdez oil spill of 1989 an estimated 247 were killed in Prince William Sound, though the local population returned to its pre-spill level by 1995.[16] In some areas, the increase in eagles has led to decreases in other bird populations[206] and the eagles may be considered a pest.[207]

    Killing permits

    In December 2016, the U.S. Fish and Wildlife Service proposed extending the permits issued to wind generation companies to allow them to kill up to 4,200 bald eagles per year without facing a penalty, four times the previous number. The permits would last 30 years, six times the previous 5-year term.[208][209]

    In captivity

    Lady Baltimore, a bald eagle in Alaska who survived a poaching attempt, in her Juneau Raptor Center mews, on August 15, 2015

    Permits are required to keep bald eagles in captivity in the United States. Permits are primarily issued to public educational institutions, and the eagles that they show are permanently injured individuals that cannot be released to the wild. The facilities where eagles are kept must be equipped with adequate caging, as well as workers experienced in the handling and care of eagles.[210] The bald eagle can be long-lived in captivity if well cared for, but does not breed well even under the best conditions.[211]

    In Canada[212] and in England[213] a license is required to keep bald eagles for falconry.[214] Bald eagles cannot legally be kept for falconry in the United States, but a license may be issued in some jurisdictions to allow use of such eagles in birds-of-prey flight shows.[215][216]

    Cultural significance

    The bald eagle is important in various Native American cultures and, as the national symbol of the United States, is prominent in seals and logos, coinage, postage stamps, and other items relating to the U.S. federal government.

    Role in Native American culture

    The bald eagle is a sacred bird in some North American cultures, and its feathers, like those of the golden eagle, are central to many religious and spiritual customs among Native Americans. Eagles are considered spiritual messengers between gods and humans by some cultures.[217] Many pow wow dancers use the eagle claw as part of their regalia as well. Eagle feathers are often used in traditional ceremonies, particularly in the construction of regalia worn and as a part of fans, bustles and head dresses. In the Navajo tradition an eagle feather is represented to be a protector, along with the feather Navajo medicine men use the leg and wing bones for ceremonial whistles.[218] The Lakota, for instance, give an eagle feather as a symbol of honor to person who achieves a task. In modern times, it may be given on an event such as a graduation from college.[219] The Pawnee consider eagles as symbols of fertility because their nests are built high off the ground and because they fiercely protect their young.[220] The Choctaw consider the bald eagle, who has direct contact with the upper world of the sun, as a symbol of peace.[221]

    Staff at the National Eagle Repository processing a bald eagle

    During the Sun Dance, which is practiced by many Plains Indian tribes, the eagle is represented in several ways. The eagle nest is represented by the fork of the lodge where the dance is held. A whistle made from the wing bone of an eagle is used during the course of the dance. Also during the dance, a medicine man may direct his fan, which is made of eagle feathers, to people who seek to be healed. The medicine man touches the fan to the center pole and then to the patient, in order to transmit power from the pole to the patient. The fan is then held up toward the sky, so that the eagle may carry the prayers for the sick to the Creator.[222]

    Current eagle feather law stipulates that only individuals of certifiable Native American ancestry enrolled in a federally recognized tribe are legally authorized to obtain or possess bald or golden eagle feathers for religious or spiritual use. The constitutionality of these laws has been questioned by Native American groups on the basis that it violates the First Amendment by affecting ability to practice their religion freely.[223][224]

    The National Eagle Repository, a division of the FWS, exists as a means to receive, process, and store bald and golden eagles which are found dead and to distribute the eagles, their parts and feathers to federally recognized Native American tribes for use in religious ceremonies.[225]

    National symbol of the United States

    Further information: Great Seal of the United States § Obverse

    Seal of the president of the United States

    The bald eagle is the national symbol of the United States.[226] It was adopted as a national emblem in 1782, but not designated the “national bird” until an act of Congress in December 2024.[227][228]

    The founders of the United States were fond of comparing their new republic with the Roman Republic, in which eagle imagery (usually involving the golden eagle) was prominent. On June 20, 1782, the Continental Congress adopted the design for the Great Seal of the United States, depicting a bald eagle grasping 13 arrows and an olive branch with thirteen leaves with its talons.[229][230][231]

    The bald eagle appears on most official seals of the U.S. government, including the presidential seal, the presidential flag, and in the logos of many U.S. federal agencies. Between 1916 and 1945, the presidential flag (but not the seal) showed an eagle facing to its left (the viewer’s right), which gave rise to the urban legend that the flag is changed to have the eagle face towards the olive branch in peace, and towards the arrows in wartime.[232]

    Contrary to popular legend, there is no evidence that Benjamin Franklin ever publicly supported the wild turkey (Meleagris gallopavo), rather than the bald eagle, as a symbol of the United States. However, in a letter written to his daughter in 1784 from Paris, criticizing the Society of the Cincinnati, he stated his personal distaste for the bald eagle’s behavior. In the letter Franklin states:[233]

    For my own part. I wish the bald eagle had not been chosen the representative of our country. He is a bird of bad moral character. He does not get his living honestly … besides he is a rank coward: The little king bird not bigger than a sparrow attacks him boldly and drives him out of the district.

    Franklin opposed the creation of the Society because he viewed it, with its hereditary membership, as a noble order unwelcome in the newly independent Republic, contrary to the ideals of Lucius Quinctius Cincinnatus, for whom the Society was named. His reference to the two kinds of birds is interpreted as a satirical comparison between the Society of the Cincinnati and Cincinnatus.[234]

    Largely because of its role as a symbol of the United States, but also because of its being a large predator, the bald eagle has many representations in popular culture. In film and television depictions the call of the red-tailed hawk, which is much louder and more powerful, is often substituted for bald eagles.

  • Bats 

    Bats are flying mammals of the order Chiroptera (/kaɪˈrɒptərə/).[a] With their forelimbs adapted as wings, they are the only mammals capable of true and sustained flight. Bats are more agile in flight than most birds, flying with their very long spread-out digits covered with a thin membrane or patagium. The smallest bat, and arguably the smallest extant mammal, is Kitti’s hog-nosed bat, which is 29–34 mm (1.1–1.3 in) in length, 150 mm (5.9 in) across the wings and 2–2.6 g (0.071–0.092 oz) in mass. The largest bats are the flying foxes, with the giant golden-crowned flying fox (Acerodon jubatus) reaching a weight of 1.6 kg (3.5 lb) and having a wingspan of 1.7 m (5 ft 7 in).

    The second largest order of mammals after rodents, bats comprise about 20% of all classified mammal species worldwide, with over 1,400 species. These were traditionally divided into two suborders: the largely fruit-eating megabats, and the echolocating microbats. But more recent evidence has supported dividing the order into Yinpterochiroptera and Yangochiroptera, with megabats as members of the former along with several species of microbats. Many bats are insectivores, and most of the rest are frugivores (fruit-eaters) or nectarivores (nectar-eaters). A few species feed on animals other than insects; for example, the vampire bats feed on blood. Most bats are nocturnal, and many roost in caves or other refuges; it is uncertain whether bats have these behaviours to escape predators. Bats are present throughout the world, with the exception of extremely cold regions. They are important in their ecosystems for pollinating flowers and dispersing seeds; many tropical plants depend entirely on bats for these services.

    Bats provide humans with some direct benefits, at the cost of some disadvantages. Bat dung has been mined as guano from caves and used as fertiliser. Bats consume insect pests, reducing the need for pesticides and other insect management measures. Some bats are also predators of mosquitoes, suppressing the transmission of mosquito-borne diseases. Bats are sometimes numerous enough and close enough to human settlements to serve as tourist attractions, and they are used as food across Asia and the Pacific Rim. However, fruit bats are frequently considered pests by fruit growers. Due to their physiology, bats are one type of animal that acts as a natural reservoir of many pathogens, such as rabies; and since they are highly mobile, social, and long-lived, they can readily spread disease among themselves. If humans interact with bats, these traits become potentially dangerous to humans.

    Depending on the culture, bats may be symbolically associated with positive traits, such as protection from certain diseases or risks, rebirth, or long life, but in the West, bats are popularly associated with darkness, malevolence, witchcraft, vampires, and death.

    Etymology

    [edit]

    An older English name for bats is flittermouse, which matches their name in other Germanic languages (for example German Fledermaus and Swedish fladdermus), related to the fluttering of wings. Middle English had bakke, most likely cognate with Old Swedish natbakka (‘night-bat’), which may have undergone a shift from -k- to -t- (to Modern English bat) influenced by Latin blatta, ‘moth, nocturnal insect’. The word bat was probably first used in the early 1570s.[2][3] The name Chiroptera derives from Ancient Greek: χείρ – kheír, ‘hand’[4] and πτερόν – pterón, ‘wing’.[1][5]

    Phylogeny and taxonomy

    [edit]

    The early Eocene fossil microchiropteran Icaronycteris, from the Green River Formation

    Evolution

    [edit]

    The delicate skeletons of bats do not fossilise well; it is estimated that only 12% of bat genera that lived have been found in the fossil record.[6] Most of the oldest known bat fossils were already very similar to modern microbats, such as Archaeopteropus (32 million years ago). The oldest known bat fossils include Archaeonycteris praecursor and Altaynycteris aurora (55–56 million years ago), both known only from isolated teeth.[7][8] The oldest complete bat skeleton is Icaronycteris gunnelli (52 million years ago), known from two skeletons discovered in Wyoming.[9][10] The extinct bats Palaeochiropteryx tupaiodon and Hassianycteris kumari, both of which lived 48 million years ago, are the first fossil mammals whose colouration has been discovered: both were reddish-brown.[11][12]

    Bats were formerly grouped in the superorder Archonta, along with the treeshrews (Scandentia), colugos (Dermoptera), and primates.[13] Modern genetic evidence now places bats in the superorder Laurasiatheria, with its sister taxon as Ferungulata, which includes carnivoranspangolinsodd-toed ungulates, and even-toed ungulates.[14][15][16][17][18] One study places Chiroptera as a sister taxon to odd-toed ungulates (Perissodactyla).[19]

    BoreoeutheriaEuarchontoglires (primates, treeshrews, rodents, rabbits) LaurasiatheriaEulipotyphla (hedgehogs, shrews, moles, solenodons)ScrotiferaChiroptera (bats) FereuungulataFeraePholidota (pangolins) Carnivora (cats, hyenas, dogs, bears, seals, weasels)  EuungulataPerissodactyla (horses, tapirs, rhinos) Cetartiodactyla (camels, ruminants, whales)  
    Phylogenetic tree showing Chiroptera within Laurasiatheria, with Fereuungulata as its sister taxon according to a 2013 study[18]

    The flying primate hypothesis proposed that when adaptations to flight are removed, megabats are allied to primates by anatomical features not shared with microbats and thus flight evolved twice in mammals.[20] Genetic studies have strongly supported the monophyly of bats and the single origin of mammal flight.[9][20]

    Coevolutionary evidence

    [edit]

    An independent molecular analysis trying to establish the dates when bat ectoparasites (bedbugs) evolved came to the conclusion that bedbugs similar to those known today (all major extant lineages, all of which feed primarily on bats) had already diversified and become established over 100 mya (i.e., long before the oldest records for bats, 52 mya), suggesting that they initially all evolved on non-bat hosts and “bats were colonized several times independently, unless the evolutionary origin of bats has been grossly underestimated.”[21] No analysis has provided estimates for the age of the flea lineages associated with bats. The oldest known members of a different lineage of bat ectoparasites (bat flies), however, are from roughly 20 mya, well after the origin of bats.[22] The bat-ectoparasitic earwig family Arixeniidae has no fossil record, but is not believed to originate more than 23 mya.[23]

    Inner systematics

    [edit]

    ChiropteraMegachiropteraPteropodidae (megabats) MicrochiropteraRhinolophoideaMegadermatidae (false vampire bats) Craseonycteridae (Kitti’s hog-nosed bat) Rhinopomatidae (mouse-tailed bats) Hipposideridae (Old World leaf-nosed bats) Rhinolophidae (horseshoe bats) YangochiropteraMiniopteridae (long winged bat) Noctilionidae (fisherman bats) Mormoopidae (PteronotusMystacinidae (New Zealand short-tailed bats) Thyropteridae (disc-winged bats)Furipteridae Mormoopidae (MormoopsPhyllostomidae (New World leaf-nosed bats) Molossidae (free-tailed bats) Emballonuridae (sac-winged bats) Myzopodidae (sucker-footed bats)Emballonuridae (TaphozousNatalidae (funnel-eared bats) Vespertilionidae (vesper bats) 
    Internal relationships of the Chiroptera, divided into the traditional megabat and microbat clades, according to a 2011 study[24]

    Genetic evidence indicates that megabats originated during the early Eocene, and belong within the four major lines of microbats.[18] Two new suborders have been proposed; Yinpterochiroptera includes the Pteropodidae, or megabat family, as well as the families RhinolophidaeHipposideridaeCraseonycteridaeMegadermatidae, and Rhinopomatidae.[25] Yangochiroptera includes the other families of bats (all of which use laryngeal echolocation), a conclusion supported by a 2005 DNA study.[25] A 2013 phylogenomic study supported the two new proposed suborders.[18]

    ChiropteraYangochiroptera (as above) YinpterochiropteraPteropodidae (megabats) RhinolophoideaMegadermatidae (false vampire bats) horseshoe bats and allies 
    Internal relationships of the Chiroptera, with the megabats subsumed within Yinpterochiroptera, according to a 2013 study[18]
    Giant golden-crowned flying foxAcerodon jubatus

    The 2003 discovery of an early fossil bat from the 52-million-year-old Green River FormationOnychonycteris finneyi, indicates that flight evolved before echolocative abilities.[26][27] Onychonycteris had claws on all five of its fingers, whereas modern bats have at most two claws on two digits of each hand. It also had longer hind legs and shorter forearms, similar to climbing mammals that hang under branches, such as sloths and gibbons. This palm-sized bat had short, broad wings, suggesting that it could not fly as fast or as far as later bat species. Instead of flapping its wings continuously while flying, Onychonycteris probably alternated between flaps and glides in the air.[9] This suggests that this bat did not fly as much as modern bats, but flew from tree to tree and spent most of its time climbing or hanging on branches.[28] The distinctive features of the Onychonycteris fossil also support the hypothesis that mammalian flight most likely evolved in arboreal locomotors, rather than terrestrial runners. This model of flight development, commonly known as the “trees-down” theory, holds that bats first flew by taking advantage of height and gravity to drop down on to prey, rather than running fast enough for a ground-level take off.[29][30]

    ChiropteraYangochiropteraEmballonuroideaMyzopodidaeEmballonuridaeNycteridaeNoctilionoideaMystacinidaeMormoopidaePhyllostomidaeFuripteridaeNoctilionidaeThyropteridaeVespertilionoideaNatalidaeMolossidaeMiniopteridaeCistugidaeVespertilionidaeYinpterochiropteraPteropodidaeRhinolophoideaHipposideridaeRhinolophidaeRhinonycteridaeCraseonycteridaeMegadermatidaeRhinopomatidae
    Familial relationships of bats according to a 2023 study using mitochondrial and nuclear DNA from 151 species. Two different analysis methods were used which resulted in almost identical trees, except for the position of Emballonuroidea and the relationships between Rhinolophidae, Hipposideridae, and Rhinonycteridae, which are represented as polytomies.[31]

    The molecular phylogeny was controversial, as it pointed to microbats not having a unique common ancestry, which implied that some seemingly unlikely transformations occurred. The first is that laryngeal echolocation evolved twice in bats, once in Yangochiroptera and once in the rhinolophoids.[32] The second is that laryngeal echolocation had a single origin in Chiroptera, was subsequently lost in the family Pteropodidae (all megabats), and later evolved as a system of tongue-clicking in the genus Rousettus.[33] Analyses of the sequence of the vocalization gene FoxP2 were inconclusive on whether laryngeal echolocation was lost in the pteropodids or gained in the echolocating lineages.[34] Echolocation probably first derived in bats from communicative calls. The Eocene bats Icaronycteris (52 million years ago) and Palaeochiropteryx had cranial adaptations suggesting an ability to detect ultrasound. This may have been used at first mainly to forage on the ground for insects and map out their surroundings in their gliding phase, or for communicative purposes. After the adaptation of flight was established, it may have been refined to target flying prey by echolocation.[28] Analyses of the hearing gene Prestin seem to favour the idea that echolocation developed independently at least twice, rather than being lost secondarily in the pteropodids,[35] but ontogenic analysis of the cochlea supports that laryngeal echolocation evolved only once.[36]

    Classification

    [edit]

    Further information: List of chiropterans

    Bats are placental mammals. After rodents, they are the largest order, making up about 20% of mammal species.[37] In 1758, Carl Linnaeus classified the seven bat species he knew of in the genus Vespertilio in the order Primates. Around twenty years later, the German naturalist Johann Friedrich Blumenbach gave them their own order, Chiroptera.[38] Since then, the number of described species has risen to over 1,400,[39] traditionally classified as two suborders: Megachiroptera (megabats), and Microchiroptera (microbats/echolocating bats).[40] Not all megabats are larger than microbats.[41] Several characteristics distinguish the two groups. Microbats use echolocation for navigation and finding prey, but megabats apart from those in the genus Rousettus do not.[42] Accordingly, megabats have a well-developed eyesight.[40] Megabats have a claw on the second finger of the forelimb.[43][44] The external ears of microbats do not close to form a ring; the edges are separated from each other at the base of the ear.[44] Megabats eat fruitnectar, or pollen, while most microbats eat insects; others feed on fruit, nectar, pollen, fish, frogs, small mammals, or blood.[40]

    “Chiroptera” from Ernst Haeckel‘s Kunstformen der Natur, 1904

    Below is a table chart following the bat classification of families recognized by various authors of the ninth volume of Handbook of the Mammals of the World published in 2019:[45]

    Chiroptera Blumenbach, 1779
    Yinpterochiroptera Springer, Teeling, Madsen, Stanhope & Jong, 2001
    Pteropodoidea J. E. Gray, 1821
    FamilyEnglish NameNumber of SpeciesImage Figure
    Pteropodidae J. E. Gray, 1821Old World fruit bats191
    Rhinolophoidea J. E. Gray, 1825
    FamilyEnglish NameNumber of SpeciesImage Figure
    Rhinopomatidae Bonaparte, 1838Mouse-tailed bats6
    Craseonycteridae Hill, 1974Hog-nosed bat1
    Megadermatidae H. Allen, 1864False-vampires6
    Rhinonycteridae J. E. Gray, 1866Trident bats9
    Hipposideridae Lydekker, 1891Old World leaf-nosed bats88
    Rhinolophidae J. E. Gray, 1825Horseshoe bats109
    Yangochiroptera Koopman, 1984
    Emballonuroidea Gervais in de Castelnau, 1855
    FamilyEnglish NameNumber of SpeciesImage Figure
    Nycteridae Van der Hoeven, 1855Slit-faced bats15
    Emballonuridae Gervais in de Castelnau, 1855Sheath-tailed bats54
    Noctilionoidea J. E. Gray, 1821
    FamilyEnglish NameNumber of SpeciesImage Figure
    Myzopodidae Thomas, 1904Madagascar and western sucker-footed bats2
    Mystacinidae Dobson, 1875New Zealand short-tailed bats2
    Thyropteridae Miller, 1907Disk-winged bats5
    Furipteridae J. E. Gray, 1866Smoky bat and thumbless bat2
    Noctilionidae J. E. Gray, 1821Bulldog bats2
    Mormoopidae Saussure, 1860Ghost-facednaked-backed and mustached bats18
    Phyllostomidae J. E. Gray, 1825New World leaf-nosed bats217
    Vespertilionoidea J. E. Gray, 1821
    FamilyEnglish NameNumber of SpeciesImage Figure
    Natalidae J. E. Gray, 1825Funnel-eared bats10
    Molossidae Gervais in de Castelnau, 1855Free-tailed bats126
    Miniopteridae Dobson, 1875Long-fingered and bent-wing bats38
    Cistugidae Lack et al., 2010Wing-gland bats2
    Vespertilionidae J. E. Gray, 1821Vesper bats496

    Anatomy and physiology

    [edit]

    Skull and dentition

    [edit]

    A preserved megabat showing how the skeleton fits inside its skin

    The head and teeth shape of bats can vary by species. In general, megabats have longer snouts, larger eye sockets and smaller ears, giving them a more dog-like appearance, which is the source of their nickname of “flying foxes”.[46] Among microbats, longer snouts are associated with nectar-feeding,[47] while vampire bats have reduced snouts to accommodate large incisors and canines.[48]

    Small insect-eating bats can have as many as 38 teeth, while vampire bats have only 20. Bats that feed on hard-shelled insects have fewer but larger teeth with longer canines and more robust lower jaws than species that prey on softer bodied insects. In nectar-feeding bats, the canines are long while the cheek-teeth are reduced. In fruit-eating bats, the cusps of the cheek teeth are adapted for crushing.[47] The upper incisors of vampire bats lack enamel, which keeps them razor-sharp.[48] The bite force of small bats is generated through mechanical advantage, allowing them to bite through the hardened armour of insects or the skin of fruit.[49]

    Wings and flight

    [edit]

    Main articles: Bat flight and Bat wing development

    Bats are the only mammals capable of sustained flight, as opposed to gliding, as in the flying squirrel.[50] The fastest bat, the Mexican free-tailed bat (Tadarida brasiliensis), can achieve a ground speed of 160 km/h (100 mph).[51]Duration: 9 seconds.0:09Little brown bat take off and flight

    The finger bones of bats are much more flexible than those of other mammals, owing to their flattened cross-section and to low levels of calcium near their tips.[52][53] The elongation of bat digits, a key feature required for wing development, is due to the upregulation of bone morphogenetic proteins (Bmps). During embryonic development, the gene controlling Bmp signalling, Bmp2, is subjected to increased expression in bat forelimbs – resulting in the extension of the manual digits. This crucial genetic alteration helps create the specialized limbs required for powered flight. The relative proportion of extant bat forelimb digits compared with those of Eocene fossil bats have no significant differences, suggesting that bat wing morphology has been conserved for over fifty million years.[54] During flight, the bones undergo bending and shearing stress; the bending stresses felt are smaller than in terrestrial mammals, but the shearing stress is larger. The wing bones of bats have a slightly lower breaking stress point than those of birds.[55]

    As in other mammals, and unlike in birds, the radius is the main component of the forearm. Bats have five elongated digits, which all radiate around the wrist. The thumb points forward and supports the leading edge of the wing, and the other digits support the tension held in the wing membrane. The second and third digits go along the wing tip, allowing the wing to be pulled forward against aerodynamic drag, without having to be thick as in pterosaur wings. The fourth and fifth digits go from the wrist to the trailing edge, and repel the bending force caused by air pushing up against the stiff membrane.[56] Due to their flexible joints, bats are more maneuverable and more dexterous than gliding mammals.[57]

    Wing membranes (patagia) of Townsend’s big-eared batCorynorhinus townsendii

    The wings of bats are much thinner and consist of more bones than the wings of birds, allowing bats to maneuver more accurately than the latter, and fly with more lift and less drag.[58] By folding the wings in toward their bodies on the upstroke, they save 35 percent energy during flight.[59] The membranes are delicate, tearing easily,[60] but can regrow, and small tears heal quickly.[60][61] The surface of the wings is equipped with touch-sensitive receptors on small bumps called Merkel cells, also found on human fingertips. These sensitive areas are different in bats, as each bump has a tiny hair in the center, making it even more sensitive and allowing the bat to detect and adapt to changing airflow; the primary use is to judge the most efficient speed at which to fly, and possibly also to avoid stalls.[62] Insectivorous bats may also use tactile hairs to help perform complex maneuvers to capture prey in flight.[57]

    The patagium is the wing membrane; it is stretched between the arm and finger bones, and down the side of the body to the hind limbs and tail. This skin membrane consists of connective tissueelastic fibresnervesmuscles, and blood vessels. The muscles keep the membrane taut during flight.[63] The extent to which the tail of a bat is attached to a patagium can vary by species, with some having completely free tails or even no tails.[47] The skin on the body of the bat, which has one layer of epidermis and dermis, as well as hair folliclessweat glands and a fatty subcutaneous layer, is very different from the skin of the wing membrane. Depending on the bat species the presence of hair follicles and sweat glands will vary in the patagium.[64] This patagium is an extremely thin double layer of epidermis; these layers are separated by a connective tissue center, rich with collagen and elastic fibers. In some bat species sweat glands will be present in between this connective tissue.[65] Furthermore, if hair follicles are present this supports the bat in order to adjust sudden flight maneuvers.[66][67] For bat embryos, apoptosis (programmed cell death) affects only the hindlimbs, while the forelimbs retain webbing between the digits that forms into the wing membranes.[68] Unlike birds, whose stiff wings deliver bending and torsional stress to the shoulders, bats have a flexible wing membrane that can resist only tension. To achieve flight, a bat exerts force inwards at the points where the membrane meets the skeleton, so that an opposing force balances it on the wing edges perpendicular to the wing surface. This adaptation does not permit bats to reduce their wingspans, unlike birds, which can partly fold their wings in flight, radically reducing the wing span and area for the upstroke and for gliding. Hence bats cannot travel over long distances as birds can.[56]

    Nectar- and pollen-eating bats can hover, in a similar way to hummingbirds. The sharp leading edges of the wings can create vortices, which provide lift. The vortex may be stabilized by the animal changing its wing curvatures.[69]

    Roosting and gaits

    [edit]

    Group of megabats roosting

    When not flying, bats hang upside down from their feet, a posture known as roosting.[70] The femurs are attached at the hips in a way that allows them to bend outward and upward in flight. The ankle joint can flex to allow the trailing edge of the wings to bend downwards. This does not permit many movements other than hanging or clambering up trees.[56] Most megabats roost with the head tucked towards the belly, whereas most microbats roost with the neck curled towards the back. This difference is reflected in the structure of the cervical or neck vertebrae in the two groups, which are clearly distinct.[70] Tendons allow bats to lock their feet closed when hanging from a roost. Muscular power is needed to let go, but not to grasp a perch or when holding on.[71]

    When on the ground, most bats can only crawl awkwardly. A few species such as the New Zealand lesser short-tailed bat and the common vampire bat are agile on the ground. Both species make lateral gaits (the limbs move one after the other) when moving slowly but vampire bats move with a bounding gait (all limbs move in unison) at greater speeds, the folded up wings being used to propel them forward. Vampire bats likely evolved these gaits to follow their hosts while short-tailed bats developed in the absence of terrestrial mammal competitors. Enhanced terrestrial locomotion does not appear to have reduced their ability to fly.[72]

    Internal systems

    [edit]

    Bats have an efficient circulatory system. They seem to make use of particularly strong venomotion, a rhythmic contraction of venous wall muscles. In most mammals, the walls of the veins provide mainly passive resistance, maintaining their shape as deoxygenated blood flows through them, but in bats they appear to actively support blood flow back to the heart with this pumping action.[73][74] Since their bodies are relatively small and lightweight, bats are not at risk of blood flow rushing to their heads when roosting.[75]

    Bats possess a highly adapted respiratory system to cope with the demands of powered flight, an energetically taxing activity that requires a large continuous throughput of oxygen. In bats, the relative alveolar surface area and pulmonary capillary blood volume are larger than in most other small quadrupedal mammals.[76] During flight the respiratory cycle has a one-to-one relationship with the wing-beat cycle.[77] Because of the limits of mammalian lungs, bats cannot maintain high-altitude flight.[56]

    The wings are highly vascularized membranes, the larger blood vessels visible against the light.[78]

    It takes a lot of energy and an efficient circulatory system to work the flight muscles of bats. Energy supply to the muscles engaged in flight requires about double the amount compared to the muscles that do not use flight as a means of mammalian locomotion. In parallel to energy consumption, blood oxygen levels of flying animals are twice as much as those of their terrestrially locomoting mammals. As the blood supply controls the amount of oxygen supplied throughout the body, the circulatory system must respond accordingly. Therefore, compared to a terrestrial mammal of the same relative size, the bat’s heart can be up to three times larger, and pump more blood.[79] Cardiac output is directly derived from heart rate and stroke volume of the blood;[80] an active microbat can reach a heart rate of 1000 beats per minute.[81]

    With its extremely thin membranous tissue, a bat’s wing can significantly contribute to the organism’s total gas exchange efficiency.[82] Because of the high energy demand of flight, the bat’s body meets those demands by exchanging gas through the patagium of the wing. When the bat has its wings spread it allows for an increase in surface area to volume ratio. The surface area of the wings is about 85% of the total body surface area, suggesting the possibility of a useful degree of gas exchange.[82] The subcutaneous vessels in the membrane lie very close to the surface and allow for the diffusion of oxygen and carbon dioxide.[83]

    The digestive system of bats has varying adaptations depending on the species of bat and its diet. As in other flying animals, food is processed quickly and effectively to keep up with the energy demand. Insectivorous bats may have certain digestive enzymes to better process insects, such as chitinase to break down chitin, which is a large component of insects.[84] Vampire bats, probably due to their diet of blood, are the only vertebrates that do not have the enzyme maltase, which breaks down malt sugar, in their intestinal tract. Nectivorous and frugivorous bats have more maltase and sucrase enzymes than insectivorous, to cope with the higher sugar contents of their diet.[85]

    The adaptations of the kidneys of bats vary with their diets. Carnivorous and vampire bats consume large amounts of protein and can output concentrated urine; their kidneys have a thin cortex and long renal papillae. Frugivorous bats lack that ability and have kidneys adapted for electrolyte-retention due to their low-electrolyte diet; their kidneys accordingly have a thick cortex and very short conical papillae.[85] Bats have higher metabolic rates associated with flying, which lead to an increased respiratory water loss. Their large wings are composed of the highly vascularized membranes, increasing the surface area, and leading to cutaneous evaporative water loss.[78] Water helps maintain their ionic balance in their blood, thermoregulation system, and removal of wastes and toxins from the body via urine. They are also susceptible to blood urea poisoning if they do not receive enough fluid.[86]

    The structure of the uterine system in female bats can vary by species, with some having two uterine horns while others have a single mainline chamber.[87]

    Senses

    [edit]

    Echolocation

    [edit]

    Main article: Animal echolocation § Bats

    Pipistrellus pulses

    Duration: 6 seconds.0:06

    Time-expanded recording of Pipistrellus pipistrellus bat echolocation calls and social call


    Problems playing this file? See media help.

    Feeding buzz

    Duration: 8 seconds.0:08

    Time-expanded recording of the feeding buzz of a bat homing in on its prey


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    Microbats and a few megabats emit ultrasonic sounds to produce echoes. Sound intensity of these echos are dependent on subglottic pressure. The bats’ cricothyroid muscle controls the orientation pulse frequency, which is an important function. This muscle is located inside the larynx and it is the only tensor muscle capable of aiding phonation.[88] By comparing the outgoing pulse with the returning echoes, bats can gather information on their surroundings. This allows them to detect prey in darkness.[89] Some bat calls can reach 140 decibels.[90] Microbats use their larynx to emit echolocation signals through the mouth or the nose.[91] Microbat calls range in frequency from 14,000 to well over 100,000 Hz, extending well beyond the range of human hearing (between 20 and 20,000 Hz).[92] Various groups of bats have evolved fleshy extensions around and above the nostrils, known as nose-leaves, which play a role in sound transmission.[93]

    Principle of bat echolocation: orange is the call and green is the echo.

    In low-duty cycle echolocation, bats can separate their calls and returning echoes by time. They have to time their short calls to finish before echoes return.[94] The delay of the returning echoes allows the bat to estimate the range to their prey.[92] In high-duty cycle echolocation, bats emit a continuous call and separate pulse and echo in frequency using the Doppler effect of their motion in flight. The shift of the returning echoes yields information relating to the motion and location of the bat’s prey. These bats must deal with changes in the Doppler shift due to changes in their flight speed. They have adapted to change their pulse emission frequency in relation to their flight speed so echoes still return in the optimal hearing range.[94][95]

    In addition to echolocating prey, bat ears are sensitive to sounds made by their prey, such as the fluttering of moth wings. The complex geometry of ridges on the inner surface of bat ears helps to sharply focus echolocation signals, and to passively listen for any other sound produced by the prey. These ridges can be regarded as the acoustic equivalent of a Fresnel lens, and exist in a large variety of unrelated animals, such as the aye-ayelesser galagobat-eared foxmouse lemur, and others.[96][97][98] Bats can estimate the elevation of their target using the interference patterns from the echoes reflecting from the tragus, a flap of skin in the external ear.[92]

    The tiger moth (Bertholdia trigona) can jam bat echolocation.[99][100]

    By repeated scanning, bats can mentally construct an accurate image of the environment in which they are moving and of their prey.[101] Some species of moth have exploited this, such as the tiger moths, which produces aposematic ultrasound signals to warn bats that they are chemically protected and therefore distasteful.[99][100] Moth species including the tiger moth can produce signals to jam bat echolocation. Many moth species have a hearing organ called a tympanum, which responds to an incoming bat signal by causing the moth’s flight muscles to twitch erratically, sending the moth into random evasive manoeuvres.[102][103][104]

    Vision

    [edit]

    The eyes of most microbat species are small and poorly developed, leading to poor visual acuity, but no species is blind.[105] Most microbats have mesopic vision, meaning that they can detect light only in low levels, whereas other mammals have photopic vision, which allows colour vision. Microbats may use their vision for orientation and while travelling between their roosting grounds and feeding grounds, as echolocation is effective only over short distances. Some species can detect ultraviolet (UV). As the bodies of some microbats have distinct coloration, they may be able to discriminate colours.[50][106][107][108]

    Megabat species often have eyesight as good as, if not better than, human vision. Their eyesight is adapted to both night and daylight vision, including some colour vision.[108]

    Magnetoreception

    [edit]

    Microbats make use of magnetoreception, in that they have a high sensitivity to the Earth’s magnetic field, as birds do. Microbats use a polarity-based compass, meaning that they differentiate north from south, unlike birds, which use the strength of the magnetic field to differentiate latitudes, which may be used in long-distance travel. The mechanism is unknown but may involve magnetite particles.[109][110]

    Thermoregulation

    [edit]

    Thermographic image of a bat using trapped air as insulation

    Most bats are homeothermic (having a stable body temperature), the exception being the vesper bats (Vespertilionidae), the horseshoe bats (Rhinolophidae), the free-tailed bats (Molossidae), and the bent-winged bats (Miniopteridae), which extensively use heterothermy (where body temperature can vary).[111][112] Compared to other mammals, bats have a high thermal conductivity. The wings are filled with blood vessels, and lose body heat when extended. At rest, they may wrap their wings around themselves to trap a layer of warm air. Smaller bats generally have a higher metabolic rate than larger bats, and so need to consume more food in order to maintain homeothermy.[113]

    Bats may avoid flying during the day to prevent overheating in the sun, since their dark wing-membranes absorb solar radiation. Bats may not be able to dissipate heat if the ambient temperature is too high;[114] they use saliva to cool themselves in extreme conditions.[56] Among megabats, the flying fox Pteropus hypomelanus uses saliva and wing-fanning to cool itself while roosting during the hottest part of the day.[115] Among microbats, the Yuma myotis (Myotis yumanensis), the Mexican free-tailed bat, and the pallid bat (Antrozous pallidus) cope with temperatures up to 45 °C (113 °F) by panting, salivating, and licking their fur to promote evaporative cooling; this is sufficient to dissipate twice their metabolic heat production.[116]

    Bats also possess a system of sphincter valves on the arterial side of the vascular network that runs along the edge of their wings. When fully open, these allow oxygenated blood to flow through the capillary network across the wing membrane; when contracted, they shunt flow directly to the veins, bypassing the wing capillaries. This allows bats to control how much heat is exchanged through the flight membrane, allowing them to release heat during flight. Many other mammals use the capillary network in oversized ears for the same purpose.[117]

    Torpor

    [edit]

    tricoloured bat (Perimyotis subflavus) in torpor

    Torpor, a state of decreased activity where the body temperature and metabolism decreases, is especially useful for bats, as they use a large amount of energy while active, depend upon an unreliable food source, and have a limited ability to store fat. They generally drop their body temperature in this state to 6–30 °C (43–86 °F), and may reduce their energy expenditure by 50 to 99%.[118] Tropical bats may use it to avoid predation, by reducing the amount of time spent on foraging and thus reducing the chance of being caught by a predator.[119] Megabats were generally believed to be homeothermic, but three species of small megabats, with a mass of about 50 grams (1+34 ounces), have been known to use torpor: the common blossom bat (Syconycteris australis), the long-tongued nectar bat (Macroglossus minimus), and the eastern tube-nosed bat (Nyctimene robinsoni). Torpid states last longer in the summer for megabats than in the winter.[120]

    During hibernation, bats enter a torpid state and decrease their body temperature for 99.6% of their hibernation period; even during periods of arousal, when they return their body temperature to normal, they sometimes enter a shallow torpid state, known as “heterothermic arousal”.[121] Some bats become dormant during higher temperatures to keep cool in the summer months.[122]

    Heterothermic bats during long migrations may fly at night and go into a torpid state roosting in the daytime. Unlike migratory birds, which fly during the day and feed during the night, nocturnal bats have a conflict between travelling and eating. The energy saved reduces their need to feed, and also decreases the duration of migration, which may prevent them from spending too much time in unfamiliar places, and decrease predation. In some species, pregnant individuals may not use torpor.[123][124]

    Scientists reported in January 2025 that they had discovered how bats travel hundreds of miles in the spring to give birth in warmer temperatures: they surf storm fronts.[125]

    Size

    [edit]

    The smallest bat is Kitti’s hog-nosed bat (Craseonycteris thonglongyai), which is 29–34 mm (1+18–1+38 in) long with a 150-millimetre (6 in) wingspan and weighs 2–2.6 g (116332 oz).[126] It is also arguably the smallest extant species of mammal, next to the Etruscan shrew.[127] The largest bats are a few species of Pteropus megabats and the giant golden-crowned flying fox, (Acerodon jubatus), which can weigh 1.6 kg (3+12 lb) with a wingspan of 1.7 m (5 ft 7 in).[128] Larger bats tend to use lower frequencies and smaller bats higher for echolocation; high-frequency echolocation is better at detecting smaller prey. Small prey may be absent in the diets of large bats as they are unable to detect them.[129] The adaptations of a particular bat species can directly influence what kinds of prey are available to it.[130]

    Bats are thought to contribute less than 10% of the total biomass of wild terrestrial mammals, but about 66% of all mammalian individuals.[131]

    Ecology

    [edit]

    Tent-making bats (Uroderma bilobatum) in Costa Rica

    Flight has enabled bats to become one of the most widely distributed groups of mammals.[132] Apart from the Arctic, the Antarctic and a few isolated oceanic islands, bats exist in almost every habitat on Earth.[133] Tropical areas tend to have more species than temperate ones.[134] Different species select different habitats during different seasons, ranging from seasides to mountains and deserts, but they require suitable roosts. Bat roosts can be found in hollows, crevices, foliage, and even human-made structures, and include “tents” the bats construct with leaves.[135] Megabats generally roost in trees.[136] Most microbats are nocturnal[137] and megabats are typically diurnal or crepuscular.[138][139] Microbats are known to exhibit diurnal behaviour in temperate regions during summer when there is insufficient night time to forage,[140][141] and in areas where there are few avian predators during the day.[142][143]

    In temperate areas, some microbats migrate hundreds of kilometres to winter hibernation dens;[144] others pass into torpor in cold weather, rousing and feeding when warm weather allows insects to be active.[145] Others retreat to caves for winter and hibernate for as much as six months.[145] Microbats rarely fly in rain; it interferes with their echolocation, and they are unable to hunt.[146]

    Food and feeding

    [edit]Duration: 1 minute and 1 second.1:01Bats feeding on insects over a lake

    Different bat species have different diets, including insects, nectar, pollen, fruit and even vertebrates.[147] Megabats are mostly fruit, nectar and pollen eaters.[138] Due to their small size, high-metabolism and rapid burning of energy through flight, bats must consume large amounts of food for their size. Insectivorous bats may eat over 120 percent of their body weight per day, while frugivorous bats may eat over twice their weight.[148] They can travel significant distances each night, exceptionally as much as 38.5 km (24 mi) in the spotted bat (Euderma maculatum), in search of food.[149] Bats use a variety of hunting strategies.[129] Bats get most of their water from the food they eat; many species also drink from water sources like lakes and streams, flying over the surface and dipping their tongues into the water.[150]

    The Chiroptera as a whole are in the process of losing the ability to synthesise vitamin C.[151] In a test of 34 bat species from six major families, including major insect- and fruit-eating bat families, all were found to have lost the ability to synthesise it, and this loss may derive from a common bat ancestor, as a single mutation.[152][b] At least two species of bat, the frugivorous bat (Rousettus leschenaultii) and the insectivorous bat (Hipposideros armiger), have retained their ability to produce vitamin C.[153]

    Insects

    [edit]

    Most microbats, especially in temperate areas, prey on insects.[147] The diet of an insectivorous bat may span many species,[154] including fliesmosquitosbeetles, moths, grasshopperscricketstermitesbeeswaspsmayflies and caddisflies.[47][155][156] Large numbers of Mexican free-tailed bats (Tadarida brasiliensis) fly hundreds of metres above the ground in central Texas to feed on migrating moths.[157] Species that hunt insects in flight, like the little brown bat (Myotis lucifugus), may catch an insect in mid-air with the mouth, and eat it in the air or use their tail membranes or wings to scoop up the insect and carry it to the mouth.[158][159] The bat may also take the insect back to its roost and eat it there.[160] Slower moving bat species, such as the brown long-eared bat (Plecotus auritus) and many horseshoe bat species, may take or glean insects from vegetation or hunt them from perches.[47] Insectivorous bats living at high latitudes have to consume prey with higher energetic value than tropical bats.[161]

    Fruit and nectar

    [edit]

    An Egyptian fruit bat (Rousettus aegyptiacus) carrying a fig

    Fruit eating, or frugivory, is found in both major suborders. Bats prefer ripe fruit, pulling it off the trees with their teeth. They fly back to their roosts to eat the fruit, sucking out the juice and spitting the seeds and pulp out onto the ground. This helps disperse the seeds of these fruit trees, which may take root and grow where the bats have left them, and many species of plants depend on bats for seed dispersal.[162][163] The Jamaican fruit bat (Artibeus jamaicensis) has been recorded carrying fruits weighing 3–14 g (1812 oz) or even as much as 50 g (1+34 oz).[164]

    Nectar-eating bats have acquired specialised adaptations. These bats possess long muzzles and long, extensible tongues covered in fine bristles that aid them in feeding on particular flowers and plants.[163][165] The tube-lipped nectar bat (Anoura fistulata) has the longest tongue of any mammal relative to its body size. This is beneficial to them in terms of pollination and feeding. Their long, narrow tongues can reach deep into the long cup shape of some flowers. When the tongue retracts, it coils up inside the rib cage.[165] Because of these features, nectar-feeding bats cannot easily turn to other food sources in times of scarcity, making them more prone to extinction than other types of bat.[166][167] Nectar feeding also aids a variety of plants, since these bats serve as pollinators, as pollen gets attached to their fur while they are feeding. Around 500 species of flowering plant rely on bat pollination and thus tend to open their flowers at night.[163] Many rainforest plants depend on bat pollination.[168]

    Vertebrates

    [edit]

    The greater noctule bat (Nyctalus lasiopterus) uses its large teeth to catch birds.[169]

    Some bats prey on other vertebrates, such as fish, frogs, lizards, birds and mammals.[47][170] The fringe-lipped bat (Trachops cirrhosus,) for example, is skilled at catching frogs. These bats locate large groups of frogs by tracking their mating calls, then plucking them from the surface of the water with their sharp canine teeth.[171] The greater noctule bat can catch birds in flight.[169] Some species, like the greater bulldog bat (Noctilio leporinus) hunt fish. They use echolocation to detect small ripples on the water’s surface, swoop down and use specially enlarged claws on their hind feet to grab the fish, then take their prey to a feeding roost and consume it.[172] At least two species of bat are known to feed on other bats: the spectral bat (Vampyrum spectrum), and the ghost bat (Macroderma gigas).[173]

    Blood

    [edit]

    The common vampire bat (Desmodus rotundus) feeds on blood (hematophagy).

    A few species, specifically the common, white-winged, and hairy-legged vampire bats, feed only on animal blood (hematophagy). The common vampire bat typically feeds on large mammals such as cattle; the hairy-legged and white-winged vampires feed on birds.[174] Vampire bats target sleeping prey and can detect deep breathing.[175] Heat sensors in the nose help them to detect blood vessels near the surface of the skin.[176] They pierce the animal’s skin with their teeth, biting away a small flap,[177] and lap up the blood with their tongues, which have lateral grooves adapted to this purpose.[178] The blood is kept from clotting by an anticoagulant in the saliva.[177]

    Predators, parasites, and diseases

    [edit]

    Further information: Bat virome

    Bats are subject to predation from birds of prey, such as owlshawks, and falcons, and at roosts from terrestrial predators able to climb, such as cats.[179] Low-flying bats are vulnerable to crocodiles.[180] Twenty species of tropical New World snakes are known to capture bats, often waiting at the entrances of refuges, such as caves, for bats to fly past.[181] J. Rydell and J. R. Speakman argue that bats evolved nocturnality during the early and middle Eocene period to avoid predators.[179] The evidence is thought by some zoologists to be equivocal so far.[182]

    little brown bat with white nose syndrome

    As are most mammals, bats are hosts to a number of internal and external parasites.[183] Among ectoparasites, bats carry fleas and mites, as well as specific parasites such as bat bugs and bat flies (Nycteribiidae and Streblidae).[184][185] Bats are among the few non-aquatic mammalian orders that do not host lice, possibly due to competition from more specialised parasites that occupy the same niche.[185]

    White nose syndrome is a condition associated with the deaths of millions of bats in the Eastern United States and Canada.[186] The disease is named after a white fungusPseudogymnoascus destructans, found growing on the muzzles, ears, and wings of affected bats. The fungus is mostly spread from bat to bat, and causes the disease.[187] The fungus was first discovered in central New York State in 2006 and spread quickly to the entire Eastern US north of Florida; mortality rates of 90–100% have been observed in most affected caves.[188] New England and the mid-Atlantic states have, since 2006, witnessed entire species completely extirpated and others with numbers that have gone from the hundreds of thousands, even millions, to a few hundred or less.[189] Nova Scotia, Quebec, Ontario, and New Brunswick have witnessed identical die offs, with the Canadian government making preparations to protect all remaining bat populations in its territory.[190] Scientific evidence suggests that longer winters where the fungus has a longer period to infect bats result in greater mortality.[191][192][193] In 2014, the infection crossed the Mississippi River,[194] and in 2017, it was found on bats in Texas.[195]

    Bats are natural reservoirs for a large number of zoonotic pathogens,[196] including rabies, endemic in many bat populations,[197][198][199] histoplasmosis both directly and in guano,[200] Nipah and Hendra viruses,[201][202] and possibly the ebola virus,[203][204] whose natural reservoir is yet unknown.[205][206] Their high mobility, broad distribution, long life spans, substantial sympatry (range overlap) of species, and social behaviour make bats favourable hosts and vectors of disease.[207] Reviews have found different answers as to whether bats have more zoonotic viruses than other mammal groups. One 2015 review found that bats, rodents, and primates all harbored significantly more zoonotic viruses (which can be transmitted to humans) than other mammal groups, though the differences among the aforementioned three groups were not significant (bats have no more zoonotic viruses than rodents and primates).[208] Another 2020 review of mammals and birds found that the identity of the taxonomic groups did not have any impact on the probability of harboring zoonotic viruses. Instead, more diverse groups had greater viral diversity.[209]

    They seem to be highly resistant to many of the pathogens they carry, suggesting a degree of adaptation to their immune systems.[207][210][211] Their interactions with livestock and pets, including predation by vampire bats, accidental encounters, and the scavenging of bat carcasses, compound the risk of zoonotic transmission.[198] Bats are implicated in the emergence of severe acute respiratory syndrome (SARS) in China, since they serve as natural hosts for coronaviruses, several from a single cave in Yunnan, one of which developed into the SARS virus.[200][212][213] However, they neither cause nor spread COVID-19.[214]

    Behaviour and life history

    [edit]

    Social structure

    [edit]Bracken Bat Cave, home to twenty million Mexican free-tailed bats

    Some bats lead solitary lives, while others live in colonies of more than a million.[215] For instance, the Mexican free-tailed bat fly for more than one thousand miles to the 100-foot (30 m) wide cave known as Bracken Cave every March to October which plays home to an astonishing twenty million of the species,[216] whereas a mouse-eared bat lives an almost completely solitary life.[217] Living in large colonies lessens the risk to an individual of predation.[47] Temperate bat species may swarm at hibernation sites as autumn approaches. This may serve to introduce young to hibernation sites, signal reproduction in adults and allow adults to breed with those from other groups.[218]

    Several species have a fission-fusion social structure, where large numbers of bats congregate in one roosting area, along with breaking up and mixing of subgroups. Within these societies, bats are able to maintain long-term relationships.[219] Some of these relationships consist of matrilineally related females and their dependent offspring.[220] Food sharing and mutual grooming may occur in certain species, such as the common vampire bat (Desmodus rotundus), and these strengthen social bonds.[221][222] Homosexual fellatio has been observed in the Bonin flying fox Pteropus pselaphon[223] and the Indian flying fox Pteropus medius,[224] though the function and purpose of this behaviour is not clear.

    Communication

    [edit]

    Acoustics of the songs of Mexican free-tailed bats[225]

    Bats are among the most vocal of mammals and produce calls to attract mates, find roost partners and defend resources. These calls are typically low-frequency and can travel long distances.[47][226] Mexican free-tailed bats are one of the few species to “sing” like birds. Males sing to attract females. Songs have three phrases: chirps, trills and buzzes, the former having “A” and “B” syllables. Bat songs are highly stereotypical but with variation in syllable number, phrase order, and phrase repetitions between individuals.[225] Among greater spear-nosed bats (Phyllostomus hastatus), females produce loud, broadband calls among their roost mates to form group cohesion. Calls differ between roosting groups and may arise from vocal learning.[227]

    In a study on captive Egyptian fruit bats, 70% of the directed calls could be identified by the researchers as to which individual bat made it, and 60% could be categorised into four contexts: squabbling over food, jostling over position in their sleeping cluster, protesting over mating attempts and arguing when perched in close proximity to each other. The animals made slightly different sounds when communicating with different individual bats, especially those of the opposite sex.[228] In the highly sexually dimorphic hammer-headed bat (Hypsignathus monstrosus), males produce deep, resonating, monotonous calls to attract females. Bats in flight make vocal signals for traffic control. Greater bulldog bats honk when on a collision course with each other.[226]

    Bats also communicate by other means. Male little yellow-shouldered bats (Sturnira lilium) have shoulder glands that produce a spicy odour during the breeding season. Like many other species, they have hair specialised for retaining and dispersing secretions. Such hair forms a conspicuous collar around the necks of the some Old World megabat males. Male greater sac-winged bats (Saccopteryx bilineata) have sacs in their wings in which they mix body secretions like saliva and urine to create a perfume that they sprinkle on roost sites, a behaviour known as “salting”. Salting may be accompanied by singing.[226]

    Reproduction and life cycle

    [edit]

    Group of polygynous vampire bats

    Most bat species are polygynous, where males mate with multiple females. Male pipistrelle, noctule and vampire bats may claim and defend resources that attract females, such as roost sites, and mate with those females. Males unable to claim a site are forced to live on the periphery where they have less reproductive success.[229][47] Promiscuity, where both sexes mate with multiple partners, exists in species like the Mexican free-tailed bat and the little brown bat.[230][231] There appears to be bias towards certain males among females in these bats.[47] In a few species, such as the yellow-winged bat and spectral bat, adult males and females form monogamous pairs.[47][232] Lek mating, where males aggregate and compete for female choice through display, is rare in bats[233] but occurs in the hammerheaded bat.[234]

    For temperate living bats, mating takes place in late summer and early autumn.[235] Tropical bats may mate during the dry season.[236] After copulation, the male may leave behind a mating plug to block the sperm of other males and thus ensure his paternity.[237] In hibernating species, males are known to mate with females in torpor.[47] Female bats use a variety of strategies to control the timing of pregnancy and the birth of young, to make delivery coincide with maximum food ability and other ecological factors. Females of some species have delayed fertilisation, in which sperm is stored in the reproductive tract for several months after mating. Mating occurs in late summer to early autumn but fertilisation does not occur until the following late winter to early spring. Other species exhibit delayed implantation, in which the egg is fertilised after mating, but remains free in the reproductive tract until external conditions become favourable for giving birth and caring for the offspring.[238] In another strategy, fertilisation and implantation both occur, but development of the foetus is delayed until good conditions prevail. During the delayed development the mother keeps the fertilised egg alive with nutrients. This process can go on for a long period, because of the advanced gas exchange system.[239]

    Newborn common pipistrelle, Pipistrellus pipistrellus

    For temperate living bats, births typically take place in May or June in the Northern Hemisphere; births in the Southern Hemisphere occur in November and December. Tropical species give birth at the beginning of the rainy season.[240] In most bat species, females carry and give birth to a single pup per litter.[241] At birth, a bat pup can be up to 40 percent of the mother’s weight,[47] and the pelvic girdle of the female can expand during birth as the two-halves are connected by a flexible ligament.[242] Females typically give birth in a head-up or horizontal position, using gravity to make birthing easier. The young emerges rear-first, possibly to prevent the wings from getting tangled, and the female cradles it in her wing and tail membranes. In many species, females give birth and raise their young in maternity colonies and may assist each other in birthing.[243][244][242]

    Most of the care for a young bat comes from the mother. In monogamous species, the father plays a role. Allo-suckling, where a female suckles another mother’s young, occurs in several species. This may serve to increase colony size in species where females return to their natal colony to breed.[47] A young bat’s ability to fly coincides with the development of an adult body and forelimb length. For the little brown bat, this occurs about eighteen days after birth. Weaning of young for most species takes place in under eighty days. The common vampire bat nurses its offspring beyond that and young vampire bats achieve independence later in life than other species. This is probably due to the species’ blood-based diet, which is difficult to obtain on a nightly basis.[245]

    Life expectancy

    [edit]

    The bat scientist Lauri Lutsar is checking the age of the bat he is holding as part of a national monitoring program in Estonia

    The maximum lifespan of bats is three-and-a-half times longer than other mammals of similar size. Six species have been recorded to live over thirty years in the wild: the brown long-eared bat (Plecotus auritus), the little brown bat (Myotis lucifugus), the Siberian bat (Myotis sibiricus), the lesser mouse-eared bat (Myotis blythii) the greater horseshoe bat (Rhinolophus ferrumequinum), and the Indian flying fox (Pteropus giganteus).[246] One hypothesis consistent with the rate-of-living theory links this to the fact that they slow down their metabolic rate while hibernating; bats that hibernate, on average, have a longer lifespan than bats that do not.[247][248]

    Another hypothesis is that flying has reduced their mortality rate, which would also be true for birds and gliding mammals. Bat species that give birth to multiple pups generally have a shorter lifespan than species that give birth to only a single pup. Cave-roosting species may have a longer lifespan than non-roosting species because of the decreased predation in caves. A male Siberian bat was recaptured in the wild after 41 years, making it the oldest known bat.[248][249]

    Interactions with humans

    [edit]

    Main article: Human uses of bats

    Conservation

    [edit]

    See also: List of bats by population

    Conservation statuses of bats as of 2020 according to the IUCN (1,314 species in total)[250]

    1. Critically endangered (1.6%)
    2. Endangered (6.3%)
    3. Vulnerable (8.3%)
    4. Near-threatened (6.7%)
    5. Least concern (58%)
    6. Data deficient (18.4%)
    7. Extinct (0.7%)

    Groups such as the Bat Conservation International[251] aim to increase awareness of bats’ ecological roles and the environmental threats they face. This group called for Bat Appreciation Week from October 24–31 every year to promote awareness on the ecological importance of bats.[252] In the United Kingdom, all bats are protected under the Wildlife and Countryside Acts, and disturbing a bat or its roost can be punished with a heavy fine.[253] In Sarawak, Malaysia, “all bats”[254] are protected under the Wildlife Protection Ordinance 1998,[254] but species such as the hairless bat (Cheiromeles torquatus) are still eaten by the local communities.[255] Humans have caused the extinction of several species of bat in modern history, the most recent being the Christmas Island pipistrelle (Pipistrellus murrayi), which was declared extinct in 2009.[256]

    Many people put up bat houses to attract bats.[257] The 1991 University of Florida bat house is the largest occupied artificial roost in the world, with around 400,000 residents.[258] In Britain, thickwalled and partly underground World War II pillboxes have been converted to make roosts for bats,[259][260] and purpose-built bat houses are occasionally built to mitigate damage to habitat from road or other developments.[261][262] Cave gates are sometimes installed to limit human entry into caves with sensitive or endangered bat species. The gates are designed not to limit the airflow, and thus to maintain the cave’s micro-ecosystem.[263] Of the 47 species of bats found in the United States, 35 are known to use human structures, including buildings and bridges. Fourteen species use bat houses.[264]

    Bats are eaten in countries across Africa, Asia and the Pacific Rim. In some cases, such as in Guam, flying foxes have become endangered through being hunted for food.[265] There is evidence that suggests that wind turbines might create sufficient barotrauma (pressure damage) to kill bats.[266] Bats have typical mammalian lungs, which are thought to be more sensitive to sudden air pressure changes than the lungs of birds, making them more liable to fatal rupture.[267][268][269][270][271] Bats may be attracted to turbines, perhaps seeking roosts, increasing the death rate.[267] Acoustic deterrents may help to reduce bat mortality at wind farms.[272]

    The diagnosis and contribution of barotrauma to bat deaths near wind turbine blades have been disputed by other research comparing dead bats found near wind turbines with bats killed by impact with buildings in areas with no turbines.[273]

    Cultural significance

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    Francisco de GoyaThe Sleep of Reason Produces Monsters, 1797

    Since bats are mammals, yet can fly, they are considered to be liminal beings in various traditions.[274] In many cultures, including in Europe, bats are associated with darkness, death, witchcraft, and malevolence.[275] Among Native Americans such as the CreekCherokee and Apache, the bat is identified as a trickster.[276] In Tanzania, a winged batlike creature known as Popobawa is believed to be a shapeshifting evil spirit that assaults and sodomises its victims.[277] In Aztec mythology, bats symbolised the land of the dead, destruction, and decay.[278][279][280] An East Nigerian tale tells that the bat developed its nocturnal habits after causing the death of his partner, the bush-rat, and now hides by day to avoid arrest.[281]

    More positive depictions of bats exist in some cultures. In China, bats have been associated with happiness, joy and good fortune. Five bats are used to symbolise the “Five Blessings”: longevity, wealth, health, love of virtue and peaceful death.[282] The bat is sacred in Tonga and is often considered the physical manifestation of a separable soul.[283] In the Zapotec civilisation of Mesoamerica, the bat god presided over corn and fertility.[284]

    Zapotec bat god, Oaxaca, 350–500 CE

    The Weird Sisters in Shakespeare’s Macbeth used the fur of a bat in their brew.[285] In Western culture, the bat is often a symbol of the night and its foreboding nature. The bat is a primary animal associated with fictional characters of the night, both villainous vampires, such as Count Dracula and before him Varney the Vampire,[286] and heroes, such as the DC Comics character Batman.[287] Kenneth Oppel‘s Silverwing novels narrate the adventures of a young bat,[288] based on the silver-haired bat of North America.[289]

    The bat is sometimes used as a heraldic symbol in Spain and France, appearing in the coats of arms of the towns of ValenciaPalma de MallorcaFragaAlbacete, and Montchauvet.[290][291][292] Three US states have an official state bat. Texas and Oklahoma are represented by the Mexican free-tailed bat, while Virginia is represented by the Virginia big-eared bat (Corynorhinus townsendii virginianus).[293]

    Economics

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    Insectivorous bats in particular are especially helpful to farmers, as they control populations of agricultural pests and reduce the need to use pesticides. It has been estimated that bats save the agricultural industry of the United States anywhere from $3.7 billion to $53 billion per year in pesticides and damage to crops. This also prevents the overuse of pesticides, which can pollute the surrounding environment, and may lead to resistance in future generations of insects.[294]

    Bat dung, a type of guano, is rich in nitrates and is mined from caves for use as fertiliser.[295] During the US Civil Warsaltpetre was collected from caves to make gunpowder. At the time, it was believed that the nitrate all came from the bat guano, but it is now known that most of it is produced by nitrifying bacteria.[296]

    The Congress Avenue Bridge in Austin, Texas, is the summer home to North America’s largest urban bat colony, an estimated 1,500,000 Mexican free-tailed bats. About 100,000 tourists a year visit the bridge at twilight to watch the bats leave the roost.