Bald eagles have adult plumage when they are 5 years old. Before that, these birds go through stages of maturity and their age is easy to determine based on their plumage coloration. Right after hatching, bald eagles have dark eyes, with pink legs and skin and flesh colored talons, their skin darkens to a bluish hue and their legs become yellow within the first 18 to 22 days of their life. Throughout their first year, their bodies, eyes and beaks are dark brown, although their underwing covert and axillary feathers are white. In their 2nd year, their eyes lighten, becoming grayish-brown, they develop a light colored superciliary line and their body becomes mottled white. During their 3rd year, these birds begin to develop yellow bills and eyes and the coloration of their head feathers lighten, although their body remains mottled. In their 4th year, their body becomes mostly dark and their head and tail become mostly white, with some beige around their eyes and crown and isolated dark spots on their tail. Immature bald eagles are often confused with golden eagles due to their dark coloration. However, immature bald eagles have a blotchy white coloration on their underwing covert, axillary and tail feathers and have longer heads and shorter tails. (Alderfer, 2006; Bortolotti, 1984a; Bortolotti, 1984b; Dickinson, 1991; Sibley, 2003)
Adult bald eagles are extremely large birds with yellow eyes and bills, white heads and tails and dark brown bodies, which may appear almost black. They get their adult plumage when they are 5 years old, but they may still have a few dark spots on their head and tail for several more years. Male and female bald eagles have the same color plumage, but females are usually a little bit larger. These birds have extremely large, powerful bodies; their plank-like wings have a span of 178 to 229 cm, their bodies are 79 to 94 cm long and they weigh about 4.3 kg. Their plumage alone weighs about 700 grams, which is twice as much as their skeleton, if lost; their flight feathers may take 2 to 3 years to replace. These birds also have large heads, necks, bills and feet with sharp talons. (Alderfer, 2006; Bortolotti, 1984a; Crossley, 2011; Dickinson, 1991; Gill, 2007; Kaufman, 2000; Sibley, 2003)
Bald eagles (Haliaeetus leucocephalus) are found throughout North America, near large water sources. These birds are native to Canada, the United States, portions of Mexico and several islands including Saint Pierre and Miquelon. Large populations of bald eagles are found in Florida, Alaska, the Pacific Northwest and near some rivers and lakes in the Midwest. These birds have small populations in Mexico, Arizona, New Mexico, Rhode Island and Vermont. They may travel through places like Belize, Bermuda, Puerto Rico and the Virgin Islands, but they do not live there permanently. Bald eagles have also been sighted in Ireland, Sweden, Siberia, Greenland and northeastern Asia. (Alderfer, 2006; Birdlife International, 2012; Buehler, 2000; Curnutt and Robertson Jr, 1994; Dickinson, 1991; Gill, 2007; Kaufman, 2000)
Bald eagles are usually found near large bodies of water such as sea coasts, coastal estuaries and inland lakes and rivers. They usually choose a habitat away from humans, with tall trees and plenty of food available. Bald eagles go out of their way to avoid humans and will even choose not to eat if their foraging areas are being disturbed by humans. Most populations, especially the ones from northern areas, migrate to southern, milder climates each year. Their nests are usually found in the canopy of tall, coniferous trees, surrounded by smaller trees, however, they may also nest in mangroves, on the ground, on cliffs, on cellular phone towers, on electrical poles and in artificial nesting towers. In the Chesapeake Bay area, these birds often roost in oak trees (Quercus) and yellow poplars (Liriodendron tulipifera), generally in woodlots with good canopy cover, although their large body size prevents their movement through closed canopies. Due to food availability, these birds may also be spotted near dams and landfills. (Andrews and Mosher, 1982; Brown, et al., 1998; Buehler, et al., 1991; Crossley, 2011; Curnutt and Robertson Jr, 1994; Dickinson, 1991; Keister Jr, et al., 1985; Millsap, et al., 2004; Saalfeld and Conway, 2010; Sibley, 2003; Stalmaster and Kaiser, 1998)
Bald eagles are monogamous. They likely mate for life, or until a pair member dies. These birds perform amazing flight displays when they come together for the breeding season, often swooping at each other. During their cart-wheel display, the birds clasp their feet in the air and spin as they plummet towards the ground, letting go before impact. (Burnie and Wilson, 2001; Gill, 2007)
Bald eagles begin breeding when they are 5 years old. Males and females construct their nests together, about 1 to 3 months before laying their eggs. Their nests are made of sticks and can be huge because birds often reuse nests and add to it each year. The largest bald eagle nest on record was found in Florida; it weighed 2 tons and had been used for 30 years when it fell out of a tree. However, nests do not generally last that long, on average nests in southern Florida and Saskatchewan are used for 5 years and nests in Alaska are used an average of 13 years. Nest are generally located away from human settlements, near water in coniferous trees, but may also be found in mangrove trees, deciduous trees, on the ground, on cellular phone towers, on electrical poles, on cliffs and in artificial nesting towers, this may vary based on the population’s location. Different regions begin nesting at different times; in Florida, they begin building their nest in September, in Ohio they begin in February and in Alaska they begin in January. (Alderfer, 2006; Andrews and Mosher, 1982; Buehler, 2000; Burnie and Wilson, 2001; Crossley, 2011; Curnutt and Robertson Jr, 1994; Dickinson, 1991; Gill, 2007; Jenkins and Jackman, 2006; Millsap, et al., 2004; Saalfeld and Conway, 2010; Watts and Duerr, 2010)
Bald eagles usually have 1 brood of 1 to 3 eggs per year; many of their eggs do not survive, although they may have replacement clutches if needed. These birds have a low fecundity, meaning they have few offspring in a year. In California, bald eagles may have up to 36 young in their lifetime; larger males usually have more offspring than smaller males. Eggs are layed at different times in different regions. In Florida, eggs are incubated beginning in October and may last until April, whereas in Yellowstone, eggs are incubated from March until April. Northern populations tend to have shorter breeding seasons, with most birds nesting at about the same time. Their eggs are round to oval and are whitish in color. Eggs are incubated for about 35 days, after hatching; these birds have an 11 to 12 week nestling period. Bald eagles are the largest semi-altricial birds in North America and weigh about 60 grams at hatching; they may gain up to 180 grams per day. Young bald eagles leave the nest when they are between 8 and 14 weeks old, this varies based on their location, although they may still be dependent until they are 18 weeks old. (Bortolotti, 1984b; Buehler, 2000; Burnie and Wilson, 2001; Gill, 2007; Jenkins and Jackman, 2006; Millsap, et al., 2004; Wood, et al., 1998)
Offspring are cared for by both parents, although eggs are brooded by females about 3 to 7 times more often than by males. The only time that the eggs are not covered is while they are being turned or when the parents are changing positions; this usually takes less than one minute, but it may be longer in mild weather. While the young are nestling they are brooded constantly until they are about 4 weeks old and are fed 4 to 5 times per day. For the first 2 weeks after hatching, most of the food is brought to the nest by the males, but after time, females also provide much of the food. The age when the offspring fledge varies but ranges between 8 and 14 weeks. Even after fledging, immature bald eagles may still be dependent on their parents for 4 to 11 more weeks. (Bryan Jr, et al., 2005; Buehler, 2000; Burnie and Wilson, 2001; Gill, 2007; Wood, et al., 1998)
Bald eagles have long lives and low adult mortality rates, although many of their eggs do not survive. A study in Florida found that a year after fledging, immature bald eagles have a survival rate of 89% in rural habitats and 65 to 72% in suburban habitats. After their 1st year, birds have an annual survival rate of 84 to 90%, regardless of their habitat type. In northern California, adult birds tend to have a 90% annual survival rate. These birds have an estimated captive lifespan of 20 to 30 years, although one captive individual reportedly survived for 47 years. Wild bald eagles in Yellowstone have an estimated life span of 15.4 years and in Prince William Sound, wild eagles have a life span of about 19 years, with no difference between males and females. The oldest known wild bald eagle was found in Alaska and survived 28 years. Common causes of bald eagle deaths include electrocution, vehicle collisions, getting caught in legs traps, accidental poisoning, starvation, malnourishment, disease and trauma caused by violent weather. (Gill, 2007; Hancock, 1973; Jenkins and Jackman, 2006; McClelland, et al., 1994; Millsap, et al., 2004; Schempf, 1997; Travsky and Beauvais, 2004)
Different populations of bald eagles show different migratory behavior, depending on where they live. For instance, some populations, such as those from Yellowstone, only move locally to areas with greater food availability and many southern populations do not migrate at all. Birds from Canadian populations often travel south to the United States during the winter, birds from the Great Lakes area may move toward the Atlantic coast, down to the Chesapeake Bay and birds from northeastern United States and Canada may move south and inland, toward the Appalachian Mountains. When they migrate, bald eagles usually go to areas with plenty of food, especially areas with open water for hunting. While migrating, many birds use mountain ranges and rivers for navigation; the Mississippi River in particular is a major migratory corridor. Immature birds have much more erratic migratory paths and patterns. Migrating birds usually begin traveling in the late morning and go back to roosting before dark. Birds from the upper Midwest may travel anywhere from 6 to 151 days to reach their summer range and 15 to 77 days to reach their wintering range. As soon as the weather is clear enough, birds return to their nesting sites. (Alderfer, 2006; Buehler, 2000; Crossley, 2011; Mandernack, et al., 2012; McClelland, et al., 1994; Millsap, et al., 2004; Sibley, 2003)
Bald eagles are often solitary, although they pair bond during the nesting season. However, groups of bald eagles may be seen in areas where there is ample food and they may roost in large groups of up to 400 individuals. Their wings are powerful, although bald eagles often choose to soar using slow, heavy wing beats, which allow them to travel far distances. When walking, bald eagles are somewhat awkward, rocking their bodies as they move. A general time budget among bald eagles includes the percentage of time resting (91%), drinking (2.6%), scavenging (2.3%) and pirating food from others (1.8%). Generally, these birds are less active during the winter, or when winds are high, likewise, they gather less food when it is raining. During the breeding season, bald eagles become territorial; vocalizing or chasing other birds. (Alderfer, 2006; Buehler, 2000; Crossley, 2011; Elliott, et al., 2006; Keister Jr, et al., 1985; Sibley, 2003)
Their home range sizes may vary. For instance, populations in Oregon and Washington have home ranges of 6 to 47 km2, with an average of 22 km2; however, a population in Alaska has a territory radius of 0.5 km2, this is believed to be the lower limit for the species. On average, their home range size is believed to be 1 to 2 km2 and does not appear to change between breeding and non-breeding seasons. (Garrett, et al., 1993; Travsky and Beauvais, 2004)
Although they are large, powerful birds, bald eagles actually make weak, high pitched, thin vocalizations including chirps, whistles and harsh chatters. These birds have 3 main types of calls, a chatter, which sounds like ‘kwit, kwit, kwit, kwit, kee-kee-kee-kee-ker’, a wail and a peal, which is a long, high-pitched cry. When they feel threatened, bald eagles may vocalize or give visual displays such as head motions, wing motions and crouching. Breeding pairs vocalize to each other when returning to their nest and have tremendous flying displays. Their eyes are large and forward facing, giving them good binocular vision. While their sense of smell may not be very good, they avoid bad tasting food items. (Alderfer, 2006; Beletsky, 2006; Buehler, 2000; Burnie and Wilson, 2001; Gill, 2007; Hansen, 1986; Kaufman, 2000; Sibley, 2003)
Bald eagles have a fairly wide diet, they are considered opportunistic foragers, but generally prefer eating fish including rainbow trout, American eels, gizzard shads, white catfish, kokanee salmon, rock greenlings, Pacific cod, atka mackerel, large mouth bass and chum salmon, among others. Bald eagles do not dive for prey; instead, they use their strong talons to remove fish near the water surface. Bald eagles also commonly hunt adult water birds, their nestlings and their eggs including common murres, great blue herons, snow geese, Ross geese, tundra swans, northern fulmars, auklets, American coots and common loons. In the winter, they eat much more carrion and small mammal prey. Bald eagles may hunt live ground squirrels, montane voles, Norway rats and sea otter pups, among others. They also feed on the carcasses of large mammals such as elk, moose, mule deer, caribou, bison, wolves and arctic foxes. These birds sometimes live near landfills and forage in human garbage. Bald eagles often pirate food from other members of their species and other raptor species, such as ospreys. Younger and smaller birds usually hunt instead of pirating from others. When they hunt, these birds perch and observe their prey, before swooping down and lifting it from the ground with their talons. When pirating food, eagles may fly, leap or walk to snatch the food away. They forage much less when disturbed by humans. For migrating birds, food is often scarce when they first arrive to their summering grounds. Fortunately, these birds can survive without food for several days. When they do eat, bald eagles often gorge and store food in their crop for later digestion. (Alderfer, 2006; Anthony, et al., 2008; Brown, 1993; Brown, et al., 1998; Bryan Jr, et al., 2005; Buehler, 2000; Burnie and Wilson, 2001; Dickinson, 1991; Elliott, et al., 2006; Hansen, 1986; Harvey, et al., 2012; Kaufman, 2000; Korhel and Clark, 1981; McCarthy, et al., 2010; McClelland, et al., 1994; Norman, et al., 1989; Parrish, et al., 2001; Sibley, 2003; Stalmaster and Kaiser, 1998; Thompson, et al., 2005)
In some areas, bald eagles have few predators, which allows them to nest on the ground. However, in most areas their eggs and young are preyed on by magpies, gulls, ravens, crows, black bears, raccoon, bobcats, wolverines and arctic foxes. Fully grown adult birds have no known predators. (Buehler, 2000; Curnutt and Robertson Jr, 1994)
As a top predator, bald eagles impact all members of their food web. Recently, their population has declined and rebounded, which has impacted many other animals. These birds may also carry several internal parasites. (Buehler, 2000; Parrish, et al., 2001; Szabo, et al., 2004)
Bald eagles do not directly have a negative impact on humans. However, to help their population rebound, buffer zones were set around their nesting sites, which may limit human development in some areas. (Birdlife International, 2012; Millsap, et al., 2004; Stalmaster and Kaiser, 1998; Wood, et al., 1998)
Bald eagles have been the symbol of the United States since 1782. They also draw in many bird watchers and other nature enthusiasts. In 1989, it was estimated that 20 to 30 million people are involved in bird watching activities, spending approximately 20 billion dollars annually. (Burnie and Wilson, 2001; Hvenegaard, et al., 1989; Kaufman, 2000; Loomis and White, 1996)
Recently, the status of bald eagles has changed a great deal. As of June 28, 2007 these birds were removed from the protection of the Endangered Species Act where they had been listed since 1978. Their population declined in the early and mid- 1900’s because of hunting, habitat destruction and the use of insecticides, such as DDT. DDT accumulates in the body fat of animals, making it particularly dangerous for top predators like bald eagles. This chemical causes their egg shells to become brittle and many of their eggs to go unhatched. Fortunately, DDT was banned in 1972 and their population has increased dramatically. In 1963, there were about 417 pairs of bald eagles in the continental US, as of 1998, there were 5,748 pairs. Guidelines were also created for how close humans can develop land near bald eagle nests. These birds are currently listed as a species of least concern according to the IUCN Red List of Threatened Species due to their increasing population and large range. Current and future threats to this species include contamination from coal power plants, Mercury poisoning and global climate change. (Birdlife International, 2012; Carlson, et al., 2012; Gill, 2007; Grubb, et al., 1990; Harvey, et al., 2012; Millsap, et al., 2004; Rockwell, 1998; Saalfeld and Conway, 2010; Schirato and Parson, 2006; Starr and Taggart, 2006; Thompson, et al., 2005; Watts and Duerr, 2010; Watts, et al., 2006; Watts, et al., 2008; Wood, et al., 1998)
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