The
Bald Eagle (
Haliaeetus leucocephalus;
hali = sea,
aeetus = eagle,
leuco = white,
cephalis = head) is a
bird of prey found in North America. A
sea eagle, it has two known sub-species and forms a
species pair with the
White-tailed Eagle (
Haliaeetus albicilla). 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 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.
[2] Sexual maturity is attained at the age of four to five years.
Bald Eagles are not actually
bald;
the name derives from an older meaning of "white headed". The adult is
mainly brown with a white head and tail. The sexes are identical in
plumage, but females are larger than males. The
beak is large and hooked. The plumage of the immature is brown.
The Bald Eagle is both the
national bird and
national animal of the United States of America. The Bald Eagle appears on its
Seal. In the late 20th century it was on the brink of
extirpation in the continental United States. Populations recovered and the species was removed from the
U.S. federal government's list of
endangered species on July 12, 1995 and transferred to the list of
threatened species. It was removed from the List of Endangered and Threatened Wildlife in the
Lower 48 States on June 28, 2007.
Description
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.
[2] The beak, feet and
irides
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.
[3] The
beak is large and hooked, with a yellow
cere.
[4] The adult Bald Eagle is unmistakable in its native range. The closely related
African Fish Eagle (
H. vocifer)
(from far outside of the Bald Eagle's range) also has a brown body,
white head and tail, but differs from the Bald in having a white chest
and black tip to the bill.
[5]
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.
[2][3] Immature Bald Eagles are distinguishable from the
Golden Eagle (
Aquila chrysaetos),
the only other very large, non-vulturine 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.
[6]
Another distinguishing feature of the immature Bald Eagle over the
mature bird is its black, yellow-tipped beak; the mature eagle has a
fully yellow beak.
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 are not generally considered a taxonomic ally of true accipitrids.
[7] However, the
Golden Eagle, averaging 4.18 kg (9.2 lb) and 63 cm (25 in) in
wing chord length in its American race (
A. c. 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).
[5][8] 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 (
H. pelagicus), may, rarely, vagrate to coastal Alaska from Asia.
[5]
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A recording of a Bald Eagle at Yellowstone National Park
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The Bald Eagle has a body length of 70–102 cm (28–40 in). Typical
wingspan is between 1.8 and 2.3 m (5.9 and 7.5 ft) and mass is normally
between 3 and 6.3 kg (6.6 and 13.9 lb).
[5] Females are about 25% larger than males, averaging 5.6 kg (12 lb), and against the males' average weight of 4.1 kg (9.0 lb).
[2][9][10][11] The size of the bird varies by location and generally corresponds with
Bergmann's rule, since the species increases in size further away from the Equator and the tropics. The smallest specimens are those from
Florida,
where mature males may weigh as little as 2.3 kg (5.1 lb) and have a
wingspan of 1.68 m (5.5 ft). Similarly small, eagles from
South Carolina average 3.27 kg (7.2 lb) in mass and 1.88 m (6.2 ft) in wingspan.
[12] The largest eagles are from
Alaska, where large females may weigh up to 7.5 kg (17 lb) and span 2.44 m (8.0 ft) across the wings.
[4][13] A survey of adult weights in Alaska showed that females weighed on average 6.3 kg (14 lb) and males weighed 4.3 kg (9.5 lb).
[14] Among standard linear measurements, the
wing chord is 51.5–69 cm (20.3–27 in), the
tail is 23–37 cm (9.1–15 in) long, and the
tarsus is 8 to 11 cm (3.1 to 4.3 in).
[5][15] 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).
[15][16]
The call consists of weak staccato, chirping whistles,
kleek kik ik ik ik, somewhat similar in cadance to a
gull's call. The calls of young birds tend to be more harsh and shrill than those of adults.
[5][6]
Taxonomy
The Bald Eagle placed in the genus
Haliaeetus (
sea eagles) which gets both its common and specific scientific names from the distinctive appearance of the adult's head.
Bald in the
English name is derived from the word
piebald, and refers to the white head and tail feathers and their contrast with the darker body.
[17] The scientific name is derived from
Haliaeetus,
New Latin for "sea eagle" (from the
Ancient Greek haliaetos), and
leucocephalus, Latinized Ancient Greek for "white head," from λευκος
leukos ("white") and κεφαλη
kephale ("head").
[18][19]
The Bald Eagle was one of the many species originally described by
Linnaeus in his 18th century work
Systema Naturae, under the name
Falco leucocephalus.
[20]
There are two recognized
subspecies of Bald Eagle:
[2][21]
- H. l. leucocephalus (Linnaeus, 1766) is the nominate subspecies. It is separated from H. l. washingtoniensis at approximately latitude 38° N, or roughly the latitude of San Francisco.[22] It is found in the southern United States and Baja California.[23]
- 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.[2][23] This subspecies reaches further south than latitude 38° N on the Atlantic Coast, where they occur in the Cape Hatteras area.[22]
The Bald Eagle forms a
species pair with the
Eurasian
White-tailed Eagle. 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.
[24] The two species probably diverged in the North
Pacific, as the White-tailed Eagle spread westwards into Eurasia and the Bald Eagle spread eastwards into North America.
[25]
Range
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.
[26]
Today, they are much more common (almost attaining their peak numbers
pre-colonization in North America), and nest in every continental state
and province in the United States and Canada.
[27]
Bald Eagles will 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. The birds primarily gather along the
Squamish and
Cheakamus Rivers, attracted by the
salmon spawning in the area.
[28]
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 in
Kerry on November 15, 1987.
[29]
Habitat
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 km
2 (4 sq mi) are optimal for breeding Bald Eagles.
[30]
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.
[31]
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 6 m
(20 ft) above the ground.
[32] 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.
[33][34] Trees used for nesting in the
Greater Yellowstone area average 27 m (89 ft) high.
[35]
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.
[30]
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.
[7]
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 nearly 10 feet wide and 20 feet deep.
[36]
In Florida, nesting habitats often consist of
mangrove swamps, the shorelines of lakes and rivers,
pinelands, seasonally flooded
flatwoods,
hardwood 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.
[32] 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.
[7] In
Southeast Alaska,
Sitka Spruce (
Picea sitchensis) provided 78% of the nesting trees used by eagles, followed by
hemlocks (
Tsuga) at 20%.
[31] Increasingly, eagles nest in man-made reservoirs stocked with fish.
[32]
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.
[30]
However, Bald Eagles will occasionally venture into 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.
[37][38] 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.
[39]
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
prairies,
meadows or
tundra, or open forests with regular carrion access.
[7][31]
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.
[40] Its dive speed is between 120–160 km/h (75–99 mph), though it seldom dives vertically.
[41]
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.
[42] The Bald Eagle selects migration routes which take advantage of
thermals,
updrafts,
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.
[3]
Diet and feeding
The Bald Eagle is an opportunistic carnivore with the capacity to predate a great variety of prey. Throughout their range,
fish often compromise the majority of the eagle's diet.
[43]
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%.
[44] In
Southeast Alaska, fish comprise approximately 66% of the year-around diet of Bald Eagles and 78% of the prey brought to the nest by the parents.
[45] Eagles living in the
Columbia River Estuary in
Oregon were found to rely on fish for 90% of their dietary intake.
[46] In the
Pacific Northwest, spawning
trout and
salmon provide most of the Bald Eagles' diet from late summer throughout fall.
[47] Southeast Alaskan eagles largely predate
Pink salmon (
Oncorhynchus gorbuscha),
Coho salmon (
O. kisutch) and, more locally,
Sockeye salmon (
O. nerka), with
Chinook salmon (
O. tshawytscha), due to their large size (12 to 18 kg (26 to 40 lb) average adult size) probably being taken only as carrion.
[45] 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).
[45] 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%).
[46] Eagles living in the
Chesapeake Bay in
Maryland were found to subsist largely on
American gizzard shad (
Dorosoma cepedianum),
Threadfin shad (
D. petenense) and
White bass (
Morone chrysops).
[48] Floridian eagles have been reported to predate
catfish, mostly prevalently the
Brown bullhead (
Ameiurus nebulosus) and any species in the
Ictalurus genus as well as
mullet,
trout,
needlefish, and
eels.
[7][32][49] Wintering eagles on the
Platte River in
Nebraska preyed mainly on American gizzard shads and Common Carp.
[50]
From observation in the Columbia River, 58% of the fish were caught
directly by the predating eagle, 24% were scavenged as carcasses and 18%
were pirated away from other animals.
[46]
Even eagles living in relatively arid regions still typically rely primarily on fish as prey. In
Sonora,
Mexico and
Arizona, 77% and over 73%, respectively, of prey remains at the nests were from fish, largely various catfish and
Rainbow trout (
Oncorhynchus mykiss).
Prey fish targeted by Bald Eagles are often quite large. 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.
[51] At nests around
Lake Superior, the remains of fish (mostly
suckers) were found to average 35.4 cm (13.9 in) in total length.
[52]
In the Columbia River estuary, most predated by eagles were estimated
to measure between 30 and 60 cm (12 and 24 in) in length, and carp flown
with (laboriously) were up to 86 cm (34 in) in length.
[46]
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.
[48] Bald Eagles also regularly exploit water turbines which produce battered, stunned or dead fish easily consumed.
[53] 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.
[45] 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.
[44]
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.
[46][54] Exceptionally, in the
Greater Yellowstone area, birds were eaten as regularly as fish year-around, with both prey groups comprising 43% of the studied dietary intake.
[35] Preferred avian prey includes
grebes,
alcids,
ducks,
gulls,
coots,
herons,
egrets, and
geese.
[55] 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.
[7][46] American Herring Gull (
Larus smithsonianus) are the favored avian prey species for eagles living around Lake Superior.
[52] Larger waterbirds are occasionally predated as well, with wintering
Emperor Geese (
Chen canagica) and
Snow Geese (
C. caerulescens), which gather in large groups, sometimes becoming regular prey.
[15][56] Other large waterbirds hunted at least occasionally by Bald Eagles have included
Common Loons (
Gavis immer),
[57] Great Black-backed Gulls (
Larus marinus),
[58] Sandhill Cranes (
Grus canadensis),
[59] Great Blue Herons (
Ardea herodias),
[44] Canada Geese (
Branta canadensis),
[48] Brown Pelicans (
Pelecanus occidentalis),
[32] and fledging
American White Pelicans (
P. erythrorhynchos).
[60] Colony nesting
seabirds, such as
alcids,
storm petrels,
cormorants,
Northern Gannets (
Morus bassanus),
terns and
gulls,
may be especially vulnerable to predation. Due to easy accessibility
and lack of formidable nest defense by such species, Bald Eagles are
capable of predating such seabirds at all ages, from eggs to mature
adults, and can effectively cull large portions of a colony.
[61]
