Haliaeetus leucocephalus (Linnaeus, 1766)
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Basis for Listing
The bald eagle is widespread throughout Canada and portions of the United States, and two races are recognized. The northern race ranges throughout Alaska, most of Canada (except the Archipelago and Hudson Bay lowlands), and across the northern United States from the Pacific Northwest to the Great Lakes and Maine coast. The southern race is found from the Delaware Bay south to Florida and west along the Gulf Coast. Formerly, the bald eagle also ranged across southern California and the southwestern United States. The decline of the bald eagle over its entire range in the contiguous 48 states has been well documented. Environmental contamination by DDT was the primary cause of the decline, and the mechanism was the accumulation of DDT residues in fish, the major food of bald eagles. Since the banning of DDT in the United States in 1972, bald eagle populations have increased nationwide.
The bald eagle is the second largest bird of prey, with a wingspan of 1.7-2.4 m (5.6-7.9 ft.) (Buehler 2000). The white head and tail and contrasting dark brown body of adult bald eagles are diagnostic of this species. Immature and subadult bald eagles are more difficult to identify, as their head and tail are brown or mottled brown and white. Blotchy white coloration on the underside of the wings and tail of young bald eagles help distinguish them from young golden eagles (Aquila chrysaetos), which have a more sharply defined white pattern. Bald eagles attain full adult plumage is their fourth or fifth year. In flight, bald eagles can be confused with turkey vultures (Cathartes aura), but the former can be distinguished by their tendency to soar on flat, board-like wings. Turkey vultures fly with their wings in a v-shape.
Bald eagles select nest sites near lakes and rivers in forested areas where tall, large diameter trees are available for nesting (Grier and Guinn 2003). Nest trees are usually within 0.8 km (.5 mi.) of water. In Minnesota, red pines (Pinus resinosa), white pines (Pinus strobes), large eastern cottonwoods (Populus deltoides), and aspen (Populus spp.) are most often selected for nesting. Eagles have also been known to nest on artificial structures such as powerline poles and osprey (Pandion haliaetus) nest platforms, but this is not common.
Biology / Life History
Breeding bald eagles defend territories of variable size, averaging 1-2 km2 (0.4-0.8 mi.2). The average distance between nests along a shoreline is 0.2-3.1 km (0.1-1.9 mi.) (Buehler 2000). Bald eagle pairs are monogamous and may mate for life. There is evidence that mates may winter together and maintain a pair bond. Each spring, pairs perform spectacular aerial courtship displays. In the cartwheel display, for example, the pair flys to great altitude, locks talons, and tumbles back toward the ground, breaking away at the last moment to avoid collision. Other displays include chases, rolls, and dives from great heights (Buehler 2000).
Conservation / Management
Human activity is the biggest threat to the bald eagle. Although DDT is no longer a risk factor in the United States, organophosphates, heavy metals, and other pollutants continue to cause sickness or death in bald eagles. Lead poisoning is also a significant cause of mortality, and oil spills threaten eagles in coastal areas. Bald eagles are occasionally still shot or intentionally poisoned. Additionally, collisions with vehicles and power lines are a growing threat as more land is developed (Buehler 2000). Vehicle collisions have also become more frequent as the bald eagle population has increased in Minnesota, due to their habit of feeding on deer carcasses along roads. Land development in wilderness areas and along shorelines destroys breeding habitat and winter roost sites, and increased human activity may cause bald eagles to abandon nesting or winter roosting sites. However, the fact that bald eagles are now successfully nesting in proximity to humans, even in the Twin Cities metropolitan area, indicates that some bald eagles may become habituated to humans if they are not persecuted (Buehler 2000; Grier and Guinn 2003). The U.S. Fish and Wildlife Service has prepared National Bald Eagle Management Guidelines to help landowners, land managers, and others avoid disturbing bald eagles. Preservation of large diameter white pine trees, or other large diameter, tall trees in the vicinity of lakes will ensure continued nesting habitat availability for bald eagles.
Conservation Efforts in Minnesota
After state listing in 1984, the Minnesota DNR's Nongame Wildlife Program (NWP) prepared management plans for nesting areas to insure that human activities did not interfere with successful nesting. As a result, annual surveys by the NWP documented a steady population increase in the state. NWP staff have also provided technical assistance on a case-by-case when human activities threatened nesting, roosting, or wintering bald eagles, and they have facilitated the capture and transport of injured birds to The Raptor Center at the University of Minnesota. In 2005, the DNR cooperated with the U.S. Fish and Wildlife Service (USFWS) and the U.S. Geological Survey to conduct a scientifically rigorous statewide bald eagle population survey. Another survey led by the USFWS is planned for 2010.
Baker, R. J., and Y. A. Monstad. 2005. 2005 Minnesota Bald Eagle surveys. Nongame Wildlife Program, Minnesota Department of Natural Resources Report. 4 pp.
Baker, R., J. Galli, and S. Nelson. 2000. 2000 Minnesota Bald Eagle survey. Nongame Wildlife Program, Minnesota Department of Natural Resources Report. 2 pp.
Buehler, D. A. 2000. Bald Eagle (Haliaeetus leucocephalus). Number 506 in A. Poole and F. Gill, editors. The birds of North America. The Birds of North America, Inc., Philadelphia, Pennsylvania.
Grier, J.W. and J.E. Guinn. 2003. Bald eagle habitats and responses to human disturbances in Minnesota. Final report submitted to the Natural Heritage and Nongame Research Program, Minnesota Department of Natural Resources. 48 pp.
Kussman, J. V. 1977. Post-fledging behavior of the northern Bald Eagle, Haliaeetus leucocephalus, in the Chippewa National Forest, Minnesota. Ph.D. Dissertation., University of Minnesota, Minneapolis.