During the nesting period, breeding bald eagles occupy and defend territories. A territory includes an active nest and may include one or more inactive nests that are built or maintained but not used for nesting in a given year.
Bald eagles generally nest near 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, usually with a dominant view of the surrounding landscape. Rarely in Alaska, bald eagles nest 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 territories typically include at least one perch with a clear view of the water, where they forage. Eagle nests are constructed with large sticks, and may be lined with moss, grass, plant stalks, lichens, seaweed, or sod. Nests are usually about 4-6 feet in diameter and 3 feet deep, although larger nests exist.
Dates vary, but generally egg-laying begins in April in Alaska. Clutch sizes range from one to three eggs. Successful pairs usually raise one or two young, or rarely 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 dependent on their parents for food until they disperse from the nesting territory approximately 6 weeks later. The entire breeding cycle, from initial activity at a nest through the period of fledgling dependency, is about 6 months.
5 Phases of bald eagle nesting:
- courtship and nest building
- egg laying
- incubation and hatching
- early nestling period
- late nestling period
Eagle sensitivity to humans varies among these 5 phases, with eagles being most sensitive to human disturbance 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 within each nesting phase.
Relative Sensitivity of Nesting Bald Eagle to Human Activities
|PHASE||ACTIVITY||SENSITIVITY TO HUMAN ACTIVITY||COMMENTS|
|I||Courtship and Nest Building||Most sensitive period; likely to respond negatively||Most critical time period. Disturbance is manifested in nest abandonment. Bald eagles in newly established territories are more prone to abandon nest sites.|
|II||Egg laying||Very sensitive period||Human activity of even limited duration may cause nest desertion and abandonment of territory for the nesting season.|
|III||Incubation and Hatching||Very sensitive period||Adults are less likely to abandon the nest near and after hatching. However, flushed adults leave eggs and young unattended; eggs are susceptible to thermal stress (either over heating or cooling), loss of moisture, and predation; young are vulnerable to elements.|
|IV||Nestling period, 4 to 8 weeks||Moderately sensitive period||Likelihood of nest abandonment and vulnerability of the nestlings to elements gradually decreases. However, nestlings may miss feedings, which may affect their survival, or may prematurely leave the nest due to disruption,|
|V||Nestlings 8 weeks through fledging||Very sensitive period||Gaining flight capability, nestlings 8 weeks and older may flush from the nest prematurely due to disruption and die.|
Bald eagles may respond in a variety ways when they are disturbed by human activities. During the nest building period, for example, 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. 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 be malnourished, affecting their development and ultimate survival.
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 are considered disturbance under the Eagle Act and thus a violation of the Act.
Avoiding Bald Eagle Disturbance at Nest Sites
To avoid disturbing nesting bald eagles, we recommend that you (1) maintain natural forested (or vegetative) buffers around nest trees, and (2) avoid certain activities during the nesting season. The buffer areas serve to minimize visual and auditory impacts associated with human activities near nest sites.
The impact that a new human activity has on a pair of nesting eagles depends on whether the eagles can see the activity from their nest and on how tolerant the birds are to human activity, which may be evidenced by the presence of ongoing human activity near the nest. Visibility is a factor because eagles are more prone to disturbance when an activity occurs in full view. For this reason, we recommend that people locate activities farther from the nest in areas with open vistas than in areas where the view is shielded by rolling topography, trees, or other screening factors. Also, vegetative buffers should be large enough to protect existing nest trees and provide for alternative or replacement nest trees. The size and shape of effective buffers depends on topography and other characteristics surrounding the nest site. For example, in open areas where there are few or no natural forested buffers, the distance alone will serve as the buffer. Consequently, the buffers in open areas may need to be larger than for areas with denser vegetation or other natural screening.
In addition to the physical features of the landscape, appropriate buffer size may vary according to the historical tolerances of eagles to human activities in particular localities, and may also depend on the location of the nest in relation to feeding and roosting areas used by the eagles. The continued presence of nesting bald eagles in the vicinity of the existing activities indicates that eagles in that area can tolerate a greater degree of human activity than we expect from eagles in areas that experience fewer human impacts.
We recommend seasonal restriction for many temporary activities that do not involve habitat alterations (e.g. fireworks, outdoor concerts). Potential negative impacts can be avoided by restricting these kinds of activities to the non-nesting period.
For activities that include both temporary and permanent habitat disturbance (e.g., building construction), we recommend a combination of landscape buffers and seasonal restrictions.
In the fall, bald eagles in Interior Alaska begin moving to their wintering grounds as lakes and rivers begin to freeze and prey becomes limited. Wintering grounds for migratory Alaska eagles are not understood, but is suspected Interior bald eagles winter in the Intermountain West and Pacific Northwest. Eagles breeding in Coastal Alaska remain in the vicinity of their nest sites throughout the year. Immature eagles wander more widely in search of food. An abundant, readily available food supply in conjunction 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, marine invertebrates and waterfowl, often taking the dead, crippled, or otherwise vulnerable animals. Mammalian carrion is an important alternate source of food at some locations.
At night, wintering eagles may congregate at communal roost trees. The same roost trees are used for several years. Roosts 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. The use of communal roosts is poorly documented in Alaska.
Bald eagles are not as sensitive to human disturbance during migration or the winter period as they are during the nesting period. However, wintering bald eagles congregate at specific sites year-after-year for purposes of feeding and sheltering. Bald eagles rely on these established roost sites because of their proximity to sufficient food sources. Permanent landscape changes may eliminate these relied upon areas and force bald eagles to seek out other wintering roost and foraging areas. Depending on the proximity of other suitable roost or foraging areas and the condition of the affected eagles, 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.