Bald eagles typically prefer areas near large water bodies such as sea coasts, coastal estuaries and inland lakes and rivers, in many areas, these birds are found within 3 km of a water source. Although their specific habitats may vary depending on their range, habitat selection depends largely on prey availability, the availability of tall trees and the degree of human disturbance. These birds avoid human recreation areas, bald eagles will even forgo feeding if their foraging area is being disturbed by humans. Although food availability is important to habitat selection, bald eagles will inhabit areas further from foraging grounds to avoid human interaction. Most populations, specifically those in northern regions, migrate to southern, milder climates annually. Generally, these birds nest in the canopy of tall, coniferous trees, surrounded by smaller trees, however, in southern Florida, mangroves are used instead, they have also been reported nesting 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; however, 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)
These iconic birds go through 4 distinct maturation stages, each comprising one year of their life. Immediately 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 coverts and axillaries 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, their bills and eyes begin to turn yellow 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. Finally, mature coloration is reached in their 5th year. Immature bald eagles are often confused with golden eagles due to their dark coloration. These birds can be differentiated based on the blotchy white coloration found on the underwing coverts, axillaries and tails of immature bald eagles; likewise, bald eagles have longer heads and shorter tails. (Alderfer, 2006; Bortolotti, 1984a; Bortolotti, 1984b; Dickinson, 1991; Sibley, 2003)
Adult bald eagles are extremely large birds with characteristically yellow eyes and bills, white heads and tails and dark brown bodies, which may appear almost black. Although these birds obtain their adult plumage during their 5th year, they may continue to have a few dark spots on their head and tail for several additional years. Bald eagles have sexually monomorphic plumage coloration, although females generally have a somewhat larger body size. These birds have extremely large, powerful bodies; generally 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 have a monogamous mating system. These birds are believed to mate for life, or until a pair member dies. Although they do not migrate with their mate, they perform extremely demonstrative displays when they come together for the breeding season. Bald eagles perform flight displays with their mates, 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 prior to egg laying. Bald eagle nests are composed of sticks and can be massive, as birds often reuse nests for consecutive years, continually adding to it each year. The largest bald eagle nest on record was found in Florida; it was used for 30 years and weighed 2 tons 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 varies based on the population’s location. Nesting dates vary regionally; in Florida, they begin nest building 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 typically produce 1 brood of 1 to 3 eggs per season; many of their eggs do not survive, although they may have replacement clutches if needed. These birds have a low fecundity, in California, bald eagles may produce up to 36 young in their lifetime, for males, this is significantly correlated to their body size. The timing of laying varies regionally. Eggs are incubated in Florida beginning in October and may last until April, whereas in Yellowstone, eggs are incubated from March until April. Populations located further north tend to have shorter breeding seasons and more synchronous nesting periods. Their eggs are round to oval and are generally whitish; individuals in higher latitudes often produce larger eggs. Regardless of their geographic location, eggs are generally incubated for about 35 days, followed by an 11 to 12 week nestling period. Bald eagles are the largest semi-altricial birds in North America and weigh approximately 60 grams at hatching; they may gain up to 180 grams per day. Fledgling time varies geographically; these birds leave the nest when they are between 8 and 14 weeks old, although they may remain 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)
Both parents care for their offspring, although a larger burden falls on the female. Eggs are brooded by females about 3 to 7 times more frequently than by males. Eggs are generally only exposed while the parents change positions or turn the eggs, usually for less than one minute at a time, but it may be longer in mild weather. During the nestling period, young are fed 4 to 5 times per day. Nestling young are brooded constantly until they are about 4 weeks old; females are present about 90% of that time, as opposed to 50% among males. Most of the food is brought to the nest by males during the first two weeks post-hatching; eventually females also provide much of the food. The age at fledging may vary geographically based on climate and food availability, but generally ranges between 8 and 14 weeks. Even after fledging, immature bald eagles may continue their dependency on their parents for an additional 4 to 11 weeks. (Bryan Jr, et al., 2005; Buehler, 2000; Burnie and Wilson, 2001; Gill, 2007; Wood, et al., 1998)
Bald eagles are long-lived with 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. Among wild individuals, bald eagles in Yellowstone are estimated to have a 15.4 year life expectancy, whereas in Prince William Sound, wild eagles are expected to survive about 19 years, with no difference in male and female life spans. The oldest known bald eagle in the wild was found in Alaska and survived 28 years, in northern California, the oldest known eagle survived 22 years. Their deaths are often caused by anthropogenic factors such as electrocution, vehicle collisions, getting caught in legs traps and accidental poisoning. Natural causes of death include 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)
The migratory behavior of bald eagles varies across their geographic ranges. For instance, some populations, such as those from Yellowstone, only migrate locally for increased foraging opportunities and many southern populations do not migrate at all. Migratory birds from Canadian populations typically travel south to the United States during the winter, likewise, populations nesting in the Great Lakes region may move toward the Atlantic coast, down to the Chesapeake Bay and populations from northeastern United States and Canada may move south and inland, toward the Appalachian Mountains. Migratory birds congregate in areas with food abundances, specifically those areas below the freeze line with open water for hunting. Many populations use geographic landmarks for navigation, such as mountain ranges and rivers; the Mississippi River in particular is a major migratory corridor. Immature birds have much more erratic migratory paths and patterns. While migrating, birds generally soar, beginning 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. Birds return to their nesting sites at varying times, as soon as weather allows. (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 with ample prey and they may roost communally 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 especially high, likewise, precipitation has a negative impact on their foraging success. During the breeding season, bald eagles become territorial; vocalizing or chasing conspecifics. (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 oscillate between breeding and non-breeding seasons. (Garrett, et al., 1993; Travsky and Beauvais, 2004)
Contrary to popular perception, bald eagles have relatively weak, high pitched, thin vocalizations, composed of chirps, whistles and harsh chatters. These birds produce 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 used when threats are perceived. In addition, these birds may communicate threats with a series of 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. With large, forward facing eyes, bald eagles likely have very good binocular vision. Although these birds do not have an adept sense of smell, they do avoid food items that taste spoiled. (Alderfer, 2006; Beletsky, 2006; Buehler, 2000; Burnie and Wilson, 2001; Gill, 2007; Hansen, 1986; Kaufman, 2000; Sibley, 2003)
As opportunistic foragers, bald eagles have a fairly wide diet, but generally prefer fish. With such a large range, their diet may vary greatly. Bald eagles are known to eat the following fish: rainbow trout, American eels, gizzard shads, white catfish, kokanee salmon, rock greenlings, Pacific cod, atka mackerel, large mouth bass and chum salmon, among others. The speed of a river flow can greatly impact an eagles hunting success. These birds do not submerge themselves to obtain prey; instead, they use their strong talons to remove fish near the water surface. Another large component of their diet includes 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, their diets often shift to carrion and small mammal prey. Bald eagles may hunt live ground squirrels, montane voles, Norway rats and sea otter pups, among others. Likewise, these birds feed on the carrion of large mammals such as elk, moose, mule deer, caribou, bison, wolves and arctic foxes. Populations of bald eagles have also been found residing near landfills, consuming human refuse. In addition to foraging by pursuing live prey or consuming carrion, these birds often pirate food from conspecifics and other raptor species, such as ospreys. In general, younger and smaller birds choose to hunt instead of pirate. When hunting, these birds perch and observe before descending on their prey and lifting it from the ground with their talons, however, when pirating food; eagles may fly, leap or walk to snatch the food. Regardless, bald eagles tend to forage much less when disturbed by humans, at times when humans are active in foraging areas, their feeding may be reduced by as much as 35%. For many populations, their arrival to their summering grounds marks a time of minimal food availability because many of the water sources may still be frozen. Fortunately, these birds can survive without food for several days. When food is available, 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)
For some populations, bald eagles have few predators, allowing them to nest on the ground. However, their eggs and young are often preyed upon by magpies, gulls, ravens, crows, black bears, raccoon, bobcats, wolverines and arctic foxes. Fully grown adult birds are not often subject to predation. (Buehler, 2000; Curnutt and Robertson Jr, 1994)
As a top predator, bald eagles impact all members of their trophic community. Their decline and recent population resurgence has impacted many other organisms, they are even causing a population decline in common murres. These birds have been found with Toxoplasma gondii as well as a protozoan, 2 genera of trematodes, 1 genus of acanthocephalan and 7 genera of nematodes. (Buehler, 2000; Parrish, et al., 2001; Szabo, et al., 2004)
Bald eagles have been the national symbol of the United States since 1782. As a highly charismatic species, bald eagles draw bird watchers and other nature enthusiasts. In 1989, it was estimated that 20 to 30 million people are involved in bird watching activities, which may equate to approximately 20 billion dollars annually. (Burnie and Wilson, 2001; Hvenegaard, et al., 1989; Kaufman, 2000; Loomis and White, 1996)
Bald eagles do not directly have a negative impact on humans. However, as a method of habitat management, buffer zones were established around their nesting sites, which limits human development in some areas. (Birdlife International, 2012; Millsap, et al., 2004; Stalmaster and Kaiser, 1998; Wood, et al., 1998)
The conservation status of bald eagles has shifted greatly during the recent past. 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 was negatively impacted in the early and mid- 1900’s by hunting, habitat destruction and the use of insecticides, such as DDT. The Bald Eagle Protection Act was put into effect in 1940, although their populations continued to decline throughout the 1950’s and 70’s. DDT’s fat soluble properties allow it to accumulate in the fats of organisms, because it biologically magnifies, top predators, such as bald eagles, were at great risk. DDT impacts all animals, with impacts such as deformities, neurological damage and in the case of birds, brittle egg shells and unhatching eggs. Fortunately, after the ban of DDT in 1972, their population has increased dramatically. In 1963, there were an estimated 417 pairs of bald eagles remaining in the continental US, as of 1998, there were 5,748 pairs, bringing their productivity back to levels seen prior to DDT usage. In addition, their population in Alaska as of 1993 was between 20 and 25,000 individuals. In Washington State, these birds have had a 700% population increase from 1981 to 2005, growing approximately 9% annually. Other factors, such as guidelines regarding the proximity in which humans can develop near bald eagle nests have also positively impacted the species' population. Bald eagles 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)
Leila Siciliano Martina (author), Animal Diversity Web Staff.
living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.
uses sound to communicate
young are born in a relatively underdeveloped state; they are unable to feed or care for themselves or locomote independently for a period of time after birth/hatching. In birds, naked and helpless after hatching.
Referring to an animal that lives in trees; tree-climbing.
having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
an animal that mainly eats meat
flesh of dead animals.
uses smells or other chemicals to communicate
the nearshore aquatic habitats near a coast, or shoreline.
humans benefit economically by promoting tourism that focuses on the appreciation of natural areas or animals. Ecotourism implies that there are existing programs that profit from the appreciation of natural areas or animals.
animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.
an area where a freshwater river meets the ocean and tidal influences result in fluctuations in salinity.
parental care is carried out by females
forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.
offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).
parental care is carried out by males
makes seasonal movements between breeding and wintering grounds
Having one mate at a time.
having the capacity to move from one place to another.
This terrestrial biome includes summits of high mountains, either without vegetation or covered by low, tundra-like vegetation.
the area in which the animal is naturally found, the region in which it is endemic.
reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.
an animal that mainly eats fish
breeding is confined to a particular season
reproduction that includes combining the genetic contribution of two individuals, a male and a female
uses touch to communicate
that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).
uses sight to communicate
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