Bald Eagle

Map of Bald Eagle sightings

Habitat Requirements
General
Bald eagles winter primarily in the temperate zone, generally below 500 m (1,640 ft)
elevation. In Colorado, however, wintering areas may reach 2,500 m (8,200 ft) elevation.
Bald eagle winter habitat is generally defined by food availability, presence of roost sites
that provide protection from inclement weather, and absence of human disturbance. The
majority of wintering bald eagles are associated with aquatic areas with some open water
for foraging (Buehler, 2000). In some areas, however, bald eagles use habitats in winter with little or no open water if other food sources (e.g., small mammals or carrion) are readily
available (NatureServe, 2006). Type of food consumed (avian, mammalian, or fish) and
means of availability (live or carrion) vary greatly across wintering range. Winter perching
habitat is characterized by the presence of tall trees located less than 50 m (164 ft) from
foraging areas (Buehler, 2000).
Bald eagles have shown high site fidelity to wintering grounds (Buehler, 2000). In Colorado,
10 of 36 immatures and adults repeatedly returned to the same area to winter, and one
individual wintered in the same area for 10 years (Harmata and Stahlecker, 1993).

Bald eagle winter ranges, especially those of non-breeding birds, can be very large
(NatureServe, 2006). An immature bald eagle wintered in Arizona over an area of more than
40,000 square kilometers (km 2 ) (15,444 square miles [mi 2 ]) and spent the summer in the
Northwest Territories on a summer range of more than 55,000 km 2 (21,235 mi 2 ) (Grubb et al.,
1994). During February to April, the mean minimum winter home range of four immature
bald eagles in Arizona averaged 400 km 2 (154 mi 2 )(Grubb et al., 1989), and in Montana,
adults and immatures had winter ranges from 102 to 3,925 km 2 (39 to 1,515 mi 2 ) (McClelland
et al., 1996). Winter home ranges in Colorado averaged 311 km 2 (120 mi 2 ); ranges for mated
birds were less than for unmated birds (128 km 2 and 546 km 2 , respectively) (49 mi 2 and
211 mi 2 , respectively) (Harmata, 1984).

Reproduction and Mortality
Bald eagle nest-building activity and egg-laying timing vary throughout the United States
depending on latitude (Buehler, 2000). In the northern United States, including Colorado,
bald eagles begin building nests between December and mid-March, and eggs are laid from
February through April. Bald eagles lay from one to four eggs, with one or two eggs being
most common. Only one egg is laid per day, and eggs are not always laid on successive
days. Incubation begins after the first egg is laid, and hatching of young occurs on different
days, resulting in chicks of unequal size occupying the same nest. Incubation typically lasts
33 to 35 days but can be as long as 45 days. Egg hatching and young rearing take place from
March to June and by mid-May to August, the young are fledging. At 10 to 12 weeks after
hatching, eaglets make their first flights, and they fledge within a few days after that first
flight. After fledging, young birds usually remain in the vicinity of the nest for several
weeks. Young are almost completely dependent on their parents for food until
approximately 6 weeks later, when they disperse from the nesting territory. Overall, the
national fledging rate is approximately one chick per nest per year (USFWS, 2006a).

Bald eagles exhibit high nest fidelity and nesting territories are often used year after year. Generally, nests are found near coastlines, rivers, lakes, or streams that support an adequate food supply. Nests are located in mature or old-growth trees, snags, cliffs, rock promontories, but rarely on the ground and, with increasing frequency, on human-made structures, including power poles and communication towers (USFWS, 2006a). In suitable forested areas, nest trees are generally the largest trees with accessible limbs capable of holding a nest that can weigh more than 455 kg (1,000 lbs) (USFWS, 2006a; Buehler, 2000). Nests are constructed from large sticks and may be lined with moss, grass, plant stalks, lichen, seaweed, or sod. Bald eagle nests are typically 1.2 to 1.8 m (4 to 6 ft) in diameter and 0.9 m (3 ft) deep (USFWS, 2006a). Nest sites generally include at least one perch with good visibility of the surrounding area (USFWS, 2006a; Buehler, 2000).

Humans represent the single greatest cause of bald eagle mortality, including mortality from direct human actions (shooting, trapping, poisoning) and mortality related to indirect human development activities (power lines and other structures). Environmental contaminants are also a significant source of mortality. These include ingestion of lead from waterfowl, deer, and other game species’ carcasses, and secondary poisoning through consumption of prey killed by pesticides or euthanasia (sodium pentobarbital). Bald eagles are also susceptible to motor vehicle-impact injuries while scavenging carcasses off highways (USFWS, 2006a; Buehler, 2000).

Movement and Behavior
Bald eagles have a complex pattern of migration that is dependent on age of the individual (immature or adult), location of breeding site (north versus south, interior versus coastal), severity of climate (especially during winter, but also possibly during summer), and year-round food availability. Adult bald eagles migrate as needed when food becomes unavailable. Bald eagles usually migrate alone but occasionally join other migrants. Concentrations of migrants may be found at communal feeding or roosting sites. Immature bald eagles migrate and move nomadically, presumably because they are not tied to a nest site (Buehler, 2000). Bald eagles migrate widely over most of North America. Northward migration may be more rapid than the return trip south to wintering grounds because early arrival on breeding grounds provides advantages in competing for nest sites and mates. Migration southward may occur at a slower rate as birds respond to foraging opportunities along the way (Buehler, 2000).

Foraging and Diet
Bald eagles are opportunistic feeders, and fish make up most of their diet. Bald eagles also eat waterfowl, shorebirds/colonial water birds, small mammals, reptiles, amphibians, and carrion (USFWS, 2006a; Buehler, 2000). Bald eagles are visual hunters and usually locate their prey from a conspicuous perch or from soaring flight, then swoop down and strike. Large numbers of bald eagles often congregate in winter to feed on spawning salmon and other fish species or in areas below reservoirs (especially hydropower dams) where fish are abundant. In winter, bald eagles take birds from rafts of ducks on reservoirs and rivers, and congregate on melting ice sheets to scavenge dead fish. Bald eagles also eat roadkill and euthanized animal carcasses at landfills and feedlots. In addition, young eagles will often congregate to feed on easily acquired food such as carrion and fish found in abundance at the mouths of streams and shallow bays, and at landfills (USFWS, 2006a).

Population Status
The bald eagle has been extensively surveyed on breeding and wintering grounds throughout their range. In the 1980s, population estimates were from 70,000 to 80,000 birds, and populations in the 1990s undoubtedly increased (Buehler, 2000). In 1999, the entire bald eagle population was estimated to be around 100,000 individuals with the greatest numbers found in Alaska and British Columbia (Buehler, 2000). In 1963, it was estimated that the lower 48 states had less than 500 pairs of nesting bald eagles, and USFWS-coordinated surveys in 1973-1974 estimated 1,000 pairs. In 2000, the USFWS recorded more than 6,471 occupied breeding areas. The 2001 estimate for breeding pairs in Colorado was 45 (USFWS, 2006b). The estimated total wintering population of bald eagles in the continental United States was over 20,000 by 2000 (Buehler, 2000).

Species Status
The bald eagle was first listed under the Endangered Species Act (ESA) as endangered on March 11, 1967, (32 Federal Regulation [FR] 4001) and was downlisted to threatened in July 1995 (60 FR 35999 36010). Primary agents that contributed to listing the bald eagle are habitat loss and contaminants (USFWS, 2006b). Due to population rebounds, the USFWS in 1999 proposed to remove the bald eagle from the threatened and endangered species list (64 FR 36454). The public comment period for the proposal to delist the bald eagle closed on June 19, 2006 (71 FR 8238). Banning dichlorodiphenyl tricholorethane (DDT) and other harmful organochlorines from use in the United States and promulgation of the ESA with the subsequent listing of the bald eagle were the two major actions contributing to the recovery of the bald eagle. Impacts from contaminants have also been reduced through elimination of lead shot for waterfowl hunting and restrictions on other harmful pesticides. Vigorous law enforcement efforts also added to the recovery by reducing the shooting of bald eagles (USFWS, 2006b). Bald eagles are also protected by the Bald and Golden Eagle Protection Act (BGEPA) and the Migratory Bird Treaty Act (MBTA). Bald eagles are a Colorado state-listed threatened species.

Conservation Measures
The USFWS has developed and is implementing the Northern States Bald Eagle Recovery Plan, which includes Colorado (USFWS, 1983). The plan includes four basic elements:
• Determine current population and habitat status.
• Determine minimum population and habitat needed to achieve recovery.
• Protect, enhance, and increase bald eagle populations and habitats.
• Establish and implement a coordination system for information and communication. In the event the bald eagle is removed from the Federal List of Endangered and Threatened Wildlife and Plants and does not have protection under the ESA, Draft National Bald Eagle
Management Guidelines (Guidelines) have been established to promote the continued conservation of the bald eagle (USFWS, 2006a). The Guidelines are intended to:
• Publicize the provisions of the BGEPA that continue to protect bald eagles in order to reduce the possibility that people will violate the law.
• Advise landowners, land managers, and the general public of the potential for various human activities to disturb bald eagles.
• Encourage land management practices that benefit bald eagles and their habitat.

Natural Threats
Predators
Bald eagles will defend their nests against other avian species, especially ravens and other raptors. Bald eagle eggs, nestlings, and fledglings are the most vulnerable to predation. Black-billed magpies, gulls, ravens, crows, black bears, and raccoons have been reported to prey on eggs in nests. Black bears, raccoons, hawks, owls, crows, ravens, bobcats, and wolverines have been reported to kill nestlings, although there is little actual documentation. Fledglings on the ground are vulnerable to mammalian predators. Few non-human species are capable of or likely to prey on immature or adult bald eagles. Starving, injured, or diseased immatures and adults may be vulnerable to mammalian predation (Buehler, 2000).

Disease
Of 1,428 bald eagles examined during a 20-year period, only 2 percent died directly from disease. Diseases reported as leading to death included peritonitis, pneumonia, enteritis, septicemia, avian cholera, aspergillosis, hepatic necrosis, and myocardial infarction. Avian pox has been reported in a few cases, including one case involving mortality of two bald eagles (Buehler, 2000).

Parasites
Few data on parasites exist, and no parasites have been reported to cause death of an individual bald eagle. Parasites appear to be common on nestling bald eagles (Buehler, 2000).

Exposure
Although little mortality is attributed to exposure, extreme weather conditions that lead to food shortages may cause death. Bald eagles can tolerate extreme cold, wind, and snow as long as food is available (Buehler, 2000).

Black-Tailed Prairie Dog Plague Outbreaks
Black-tailed prairie dogs are an important food source for wintering bald eagles. If natural prairie dog plague outbreaks cause significant localized loss of prairie dog colonies, bald eagles may not use the area for foraging.

Human Threats
Bald eagles are sensitive to human disturbance, especially during the nesting season. During migration and winter, bald eagles often concentrate in large numbers, from hundreds to thousands of individuals, for feeding and sheltering. Bald eagles rely on established roost sites because of their proximity to sufficient food sources. Human activities near or within roost sites may prevent bald eagles from feeding or taking shelter, especially if other undisturbed or productive areas are not available. Feeding may be disrupted if there are disturbance activities in the flight path of important foraging areas. Activities that permanently alter bald eagle habitat may altogether eliminate factors essential for foraging bald eagles (USWFS, 2006a).

Military Training
There are no training restrictions or buffer zones at Fort Carson and the PCMS associated with the management of the bald eagle. Military training occurs in many forms throughout areas in which bald eagles have been found on Fort Carson and the PCMS. The most likely military training to affect bald eagles would be training that may cause prairie dog populations to decline on Fort Carson and the PCMS.

Military Training Effects on Prairie Dogs
Military training activities within prairie dog colonies, including mine plows, large-caliber
weapon firing, construction of trench obstacles, live small-arms-caliber munitions, equipment drops, and offroad vehicles, would have a direct impact on prairie dogs. Except in the smallest colonies, damage associated with this type of training would not be substantial. These activities would have a short-term adverse effect on prairie dogs and a negligible effect on the long-term viability of a colony.

Non-Military Activities
Infrastructure Construction
Construction of infrastructure, especially on Fort Carson, could have the greatest impact on existing colonies of black-tailed prairie dogs on the installations. Prairie dog burrowing activities near infrastructure may lead to human/wildlife conflicts (i.e., gnawing of electrical wiring causing malfunctions in equipment), and in these cases, prairie dogs may be controlled according to practices outlined in the Biological Assessment and Management Plan for the Black-Tailed Prairie Dog on Fort Carson and the Pinon Canyon Maneuver Site (DECAM, 2004). Loss of prairie dog populations could result in bald eagles foraging outside of the installations.

Recreation
Hunting is permitted on both Fort Carson and the PCMS. The Colorado Division of Wildlife (CDOW) sets hunting seasons, but Fort Carson and the PCMS may place additional restrictions if warranted. There is a permanent moratorium on all black-tailed prairie dog hunting on both installations. Bald eagles scavenge animal remains left by hunters, and hunting most likely increases the availability of food for bald eagles on the installations. Therefore, hunting restrictions are not warranted.

Pest Control
The Biological Assessment and Management Plan for the Black-Tailed Prairie Dog on Fort Carson and the Pinon Canyon Maneuver Site outlines approved prairie dog population-control methods. Lethal control of prairie dogs occurs on Fort Carson at sites where prairie dogs present a public health threat, threaten the safety of sanctioned Army activities, damage or threaten to damage Army property, or where their presence is incompatible with current land-use practices or management goals. No prairie dogs have ever been poisoned on the PCMS, but lethal removal of prairie dogs could be employed on the PCMS in the future under the circumstances outlined above for Fort Carson (DECAM, 2004).
Aluminum phosphide (trade name Phostoxin) is the chemical agent used to control prairie dogs. Phostoxin use is restricted to times when soil temperatures are greater than 55 degrees Fahrenheit for 72 hours and acceptable soil moisture is present. Under proper conditions, Phostoxin combines with moisture in the soil to emit carbon dioxide. Phostoxin is lethal to all other wildlife species and is not used on sites where burrowing owls or mountain plovers are present (DECAM, 2004).
Bald eagles are susceptible to secondary poisoning in prairie dog colonies. The prairie dog is an important food source for bald eagles on Fort Carson and the PCMS, especially in winter. The application of any pesticide must consider the risk of secondary poisoning to bald eagles.

Power Lines
Bald eagles are susceptible to electrocution by power lines and power poles, as demonstrated by the electrocution deaths of golden eagles along Route 1 and Route 8 on Fort Carson (DECAM, 2002). Eagle electrocutions on power lines have been documented in several states, especially in the west. Problem lines are those with wires so close together that an eagle is apt to simultaneously touch two wires while attempting to land on a power pole. The problem seems to be most severe in terrestrial habitats where few suitable natural hunting perches are available (USFWS, 1983).

Follow Applicable USFWS Guidelines for Protecting Bald Eagles
The USFWS describes management practices that land owners and planners can use to benefit bald eagles. Many of the recommendations are designed to protect bald eagle habitat and ensure against illegal take under the BGEPA (USFWS, 2006a). These recommendations include:

•Protect and preserve communal roost sites, potential nest sites, and important foraging areas. Retain mature trees and old-growth stands wherever possible, particularly within one-half mile of water.
• Avoid potentially disruptive activities and development in the eagles’ direct flight path
between their nest sites, roost sites, and important foraging areas.
• Locate long-term and permanent water-dependent facilities away from important eagle foraging areas.
• Avoid recreational and commercial boating and fishing near eagle foraging areas during peak feeding times (usually early to mid-morning and late afternoon), except where eagles have demonstrated tolerance to such activity.
• Do not use explosives within one-half mile (or within 1 mile in open areas) of communal
roosts when eagles are congregating, without prior coordination with the USFWS and CDOW.
•Locate aircraft corridors no closer than 1,000 feet vertical or horizontal distance from
communal roost sites.

Only use pesticides, herbicides, fertilizers, and other chemicals in accordance with federal and state laws and labeled instructions for their use. Identify and monitor contaminants associated with hazardous waste sites (legal or illegal), and permitted releases especially within watersheds where eagles have shown poor reproduction or where bio-accumulating contaminants have been documented. These factors present a risk of contamination to eagles and their food sources. Where feasible, site wind turbines, communication towers, and high-voltage transmission power lines away from bald eagle communal roost sites to avoid collisions. Bury utility lines along forested shorelines and roadways in new development projects. Employ industry-accepted measures to prevent birds from being electrocuted on towers and poles.
Immediately cover carcasses of euthanized animals at landfills to protect eagles from being poisoned.
Do not intentionally feed bald eagles. Artificially feeding bald eagles can disrupt their essential behavioral patterns and put them at increased risk from power lines, collisions with windows and cars, and other mortality factors.

Avoid excessive groundwater pumping and river diversion that can lead to destruction
of nest trees, roosts, and foraging areas.

Use an approved non-toxic shot when hunting waterfowl. Eagles can be poisoned by
elevated levels of lead after feeding on fish and waterfowl that have ingested lead shot
or carrion killed with lead shot.