Bald Eagle Biology
As our national symbol, the adult bald eagle with its dark brown body and distinctive white head is easily identifiable. In contrast, juvenile bald eagles have mottled brown and white plumage (feathers) and gradually acquire the classic adult plumage as they mature around age 5. Fledged juveniles are as large if not larger than adults! Bald Eagles can breed around age 5; however, most don’t breed until much older. Bald Eagles are long-lived birds, many living to 25 years or older!
Diet and Hunting
Bald eagles are opportunistic feeders, with fish comprising most of their diet. They also eat waterfowl, shorebirds, colonial waterbirds, small mammals, turtles, and carrion (dead animals – often found along roads or at landfills). Eagles are visual hunters, locating their prey from a perch or soaring flight, swooping down to strike.
Bald eagle life can be broadly categorized into two categories: breeding and non-breeding. The breeding period varies by latitude and altitude. In the Pacific Northwest, breeding begins with courtship and nest building in early January, ending when the young fledge in late mid-August. Fledged juveniles remain at or near the nest and are entirely dependent on the adults for food for approximately 6 weeks after fledging, while they are learning to hunt on their own.
Bald eagles generally nest near (within 1 mile) coastlines, rivers, and large lakes where there is an adequate food supply. They nest in mature or old growth trees, snags (dead trees), cliffs, and rock promontories. Recently, and with increasing frequency, bald eagles are nesting on artificial structures such as power poles and communication towers. In forested areas, bald eagles often select the tallest trees with limbs strong enough to support a nest that can weigh more than 1,000 pounds. Nest sites typically include at least one perch with a clear view of the water, where they forage. Bald eagle nests are constructed with large sticks, and may be lined with moss, grass, plant stalks, lichens, seaweed, or sod. Nests are usually about 4 feet in diameter and 3 feet deep, although smaller and larger nests exist.
During the nesting period, breeding bald eagles occupy and defend territories. A territory includes the active nest and may include one or more alternate nests that are built or maintained but not used for nesting in a given year. Bald eagles tend to return to the same territory year after year, which is why unoccupied nests are still protected by the Bald and Golden Eagle Act.
Dates vary (see Table below), but generally egg laying begins at the end of February in the Pacific Northwest. Clutch sizes range from one to three eggs. Successful pairs usually raise one or two young, or occasionally three per nest. Eaglets make their first unsteady flights about 10 to 12 weeks after hatching, and fledge (leave their nests) within a few days after that first flight. The time between egg laying and fledging is approximately four months. However, young birds usually remain in the vicinity of the nest for several weeks after fledging because they are almost entirely dependent on their parents for food until they learn to hunt on their own and disperse from the nest territory approximately 6 weeks later. The entire breeding cycle, from initial activity at a nest through the period of fledgling dependency, is about 8 months, which is why in the Pacific Northwest, we recognize breeding season to extend from January 1 through August 15.
Breeding (January 1 – August 15)
Eagle sensitivity to human disturbance varies with eagles being most sensitive during the courtship and nest building phase.
Sensitivity also varies among individuals within each phase. Some pairs, for example, nest successfully near human activity, while others abandon nest sites in response to activities much farther away. This variability may be related to a number of factors, including visibility of the activity, its duration and noise level, extent of the area affected by the activity, the eagle pair’s prior experiences with humans, and tolerance of the individual nesting pair. Despite this variability, the sensitivity of bald eagles can be generally described below:
Bald eagles may respond in a variety ways when they are disturbed by human activities. For example, during the nest building period eagles may inadequately construct or repair their nest, or may abandon the nest, both of which can lead to failed nesting attempts. During the incubation and hatching period, human activities may startle adults or cause them to flush from the nest. Startling can damage eggs or injure young when the adults abruptly leave the nest.
Prolonged absences of adults from their nests can jeopardize eggs or young. Depending on weather conditions, eggs may overheat or cool and fail to hatch. Young nestlings rely on their parents to provide warmth or shade, and may die from hypothermia or heat stress if adults are forced away from the nest for an extended period of time (i.e., human disturbance). Eggs and juveniles are subject to greater predation risk while they are unattended.
If human activities disrupt the adults' foraging and feeding schedule, the young may not develop healthy plumage, which can affect their ability to survive. Older nestlings may be startled by loud or intrusive human activities and prematurely jump from the nest before they are able to fly or care for themselves.
Human activities that cause any of these responses and lead to injury, a decrease in productivity, or nest abandonment could be, considered disturbance under the Eagle Act, and therefore a violation.
Roosting (November 15 – March 15)
Communal Winter Roosts
Bald eagle sensitivity to human disturbance during migration and the winter period is different than during the nesting period. Wintering bald eagles congregate at specific sites year-after-year to feed and take shelter. Bald eagles rely on these established roost sites because of their proximity to sufficient food sources, which is essential to their survival during cold or inclement weather. Permanent landscape changes may eliminate these areas they rely on and force bald eagles to seek out other wintering roost and foraging areas. Depending on the availability and condition of other suitable roost or foraging areas and the health of the affected eagles, the loss of these areas can harm bald eagles. In addition, human activities near or within communal roost sites may - although not physically alter the habitat - prevent eagles from feeding or taking shelter.
In the fall, bald eagles begin moving to their wintering grounds, with the greatest numbers migrating in late October and November when summer/spring prey sources becomes limited. Wintering bald eagles occur throughout the country but are most abundant in the West and Midwest. An abundant, readily available food supply along with one or more suitable night roost sites is the primary characteristic of winter habitat.
The majority of wintering eagles are found near open water where they feed on fish and waterfowl, often taking the dead, crippled, or otherwise vulnerable animals. Mammalian carrion is an important alternate source of food at some locations. Also, many bald eagles spend a substantial portion of the non-nesting period in terrestrial habitats far from open water, relying on prey that they can easily catch, such as small mammals, or scavenging on items such as big game or livestock. Many juvenile and adult eagles are struck by vehicles because they scavenge on carcasses along roadways.
At night, wintering eagles often congregate at communal roost sites, in some cases traveling 32 miles (20 km) or more from feeding areas to a roost site. The same roost trees are used for several years. Many are in locations that are protected from the wind by vegetation or terrain, providing a more favorable thermal environment. The use of these protected sites helps minimize the energy stress encountered by wintering birds. Communal roosting may also assist eagles in finding food and socially by interacting and possibly developing pair bonds.
The Impact of Human Activity on Foraging and Roosting Bald Eagles
Disruption, destruction, or obstruction of roosting and foraging areas can negatively affect bald eagles. Disruptive activities in or near eagle foraging areas can interfere with feeding, which reduces their chance of survival. Interference with feeding can also result in reduced productivity (number of young successfully fledged). Migrating and wintering bald eagles often congregate at specific sites to feed and take shelter. Bald eagles rely on established roost sites because they are near sufficient food sources. Roost sites are usually in mature trees where the eagles are somewhat sheltered from the wind and weather.
Human activities near or within communal roost sites may prevent eagles from feeding or taking shelter, especially if there are no other undisturbed and productive feeding and roosting sites available. Activities that permanently alter communal roost sites and important foraging areas can altogether eliminate the elements that are essential for feeding and sheltering eagles.
Where a human activity agitates or bothers roosting or foraging bald eagles to the degree that causes injury or substantially interferes with breeding, feeding, or sheltering behavior and causes, or is likely to cause, a loss of productivity or nest abandonment, the conduct of the activity constitutes a violation of the Eagle Act's prohibition against disturbing eagles. The circumstances that might result in such an outcome are difficult to predict without detailed site-specific information. If your activities may disturb roosting or foraging bald eagles, you should contact your local Fish and Wildlife Service Field Office for advice and recommendations for how to avoid such disturbance.
For more information on roosts, see our Roosts in a Nutshell document (coming soon)
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April 9, 2012