Montana Field Guide

Bald Eagle - Haliaeetus leucocephalus

With a white head and tail contrasting with a dark brown body and wings, the adult plumage of the Bald Eagle, attained at approximately 5 years of age, is unmistakable. In addition to the obvious white head and tail, other distinguishing features include the yellow bill, cere, iris, legs and feet. Second in size of North American birds of prey only to the California Condor (Gymnogyps californianus), the Bald Eagle ranges in total length from 71 to 96 cm, with an average wingspan of 168 to 244 cm and a body mass ranging from 3.0 to 6.3 kg (Buehler 2000). In general appearance the sexes are similar; with females approximately 25 percent larger than males. The plumage of the juvenile birds is much less distinct, being dark brown overall. The head, body, wings, and tail are dark brown with limited mottling on the underside of the wings and on the belly. While the legs and feet of the young bird are yellow, the bill and cere are dark gray and the iris is dark brown.

The voice of the Bald Eagle is a weak series of chirps. The vocalization is described as flat chirping, stuttering whistles, given in a halting fashion, with the immature calls generally harsher and more shrill than those of the adults (Buehler 2000, Sibley 2000).

In adult plumage, the Bald Eagle is unlikely to be confused with any other species. Juvenile Bald Eagles may be confused with Golden Eagles (Aquila chrysaetos), especially with adult Goldens. A few characteristics differentiate these two species. The Bald Eagle has unfeathered legs, while those of the Golden Eagle are feathered. During flight, the head and neck of the Bald Eagle extend to about half the length of the tail, while the Golden Eagle is considerably less. This distinction is true for all age classes of both species. The terminal tail band on the Golden Eagle is dark and well defined, especially on the juveniles. In addition, the underwing and belly of the Bald Eagle show a greater amount of white compared to the Golden Eagle, whose white feathering is restricted to the base of the flight feathers (Buehler 2000).

Montana Range

The Bald Eagle is a resident species in the forested, mountainous areas of the state. Other individuals from more northerly latitudes either winter in Montana or migrate through the state to more southerly locations. Residents generally remain in the vicinity of their breeding areas throughout the year, while some, though remaining in the state, may move to the more temperate weather of lower elevations or to other areas with higher concentrations of food (Montana Bald Eagle Working Group 1994). This is especially true of individuals that nest at higher elevations.

Congregations of migrating Bald Eagles may be evident in autumn along the north-south mountain chains with an associated abundance of food sources; numerous eagles have been observed migrating over Rogers Pass and the Bridger Mountains (Hawk Watch International 2003). Large concentrations of eagles have formerly been reported feeding on spawning kokanee (Oncorhynchus nerka) in Glacier National Park and at Canyon Ferry Reservoir, north of Helena, when spawning fish were abundant (Montana Bald Eagle Working Group 1994). In the Bozeman area, birds arrive by November 20 and leave by April 25 (Skaar 1969).

In Montana, as elsewhere, the Bald Eagle is primarily a species of riparian and lacustrine habitats (forested areas along rivers and lakes), especially during the breeding season. Important year-round habitat includes wetlands, major water bodies, spring spawning streams, ungulate winter ranges and open water areas (Bureau of Land Management 1986). Wintering habitat may include upland sites. Nesting sites are generally located within larger forested areas near large lakes and rivers where nests are usually built in the tallest, oldest, large diameter trees. Nesting site selection is dependent upon maximum local food availability and minimum disturbance from human activity (Montana Bald Eagle Working Group 1994). See the Montana Bald Eagle Management Plan (1994) for further details including home range sizes and habitat requirements of fledgling birds.

The majority of their diet is comprised of fish. Important prey for Bald Eagles are waterfowl, especially in the winter, salmonids, suckers, whitefish, carrion and small mammals and birds (Bureau of Land Management 1986).

The number of birds in January increased from about 260 in 1980 to about 450 in 1984 (Bureau of Land Management 1986). Eagles on McDonald Creek in Glacier National Park increased from a peak of less than 50 in 1939 to a peak of more than 500 (Shea 1973), and have since declined because of a drop in the number of kokanee salmon spawning on McDonald Creek. Fall/winter concentrations have been noted on the Missouri River at Canyon Ferry Dam and at Fort Peck.

The Bald Eagle breeds at approximately 5 to 6 years of age. Nests are often massive structures of branches and sticks with an interior cup lined with grass, pine needles, and plant stems (Baicich and Harrison 1997). Nests may be used year after year, resulting in huge constructions, sometimes up to 12 feet in height and 8 feet in diameter. Most nests are in timber stands, 1.2 hectares with a canopy closure less than 80%. The most common nest trees are ponderosa pine, Douglas fir and cottonwood. The eggs are white, non-glossy, short ovals averaging 71 x 54 mm in size (Baicich and Harrison 1997). The clutch, usually consisting of two eggs, but may range from one to three, is laid in March or April. Incubation, performed by both sexes, lasts about 5 weeks. Mortality for the second young to hatch is high. First flight occurs at 10 to 12.5 weeks. The young are cared for by the adults at this time and may remain around the nest for several weeks after fledging. Adults may not reproduce every year.

Breeding dates in Montana range from March to July (MBD 2003). The Montana Department of Fish, Wildlife and Parks coordinates nest monitoring annually to assess nesting success. In 2001, approximately 180 nests were examined. Nesting attempts at twenty-three nests were unsuccessful; 28 were either unoccupied during the breeding season, were occupied by another species, or the fate was unknown; and the remaining nests produced 256 fledglings (MFWP 2001, DuBois pers. comm. 2003).

General objectives of habitat management for Bald Eagles in Montana include: maintaining prey bases; maintaining forest stands currently used or suitable for nesting, roosting, and foraging; planning for future potential nesting, roosting, and foraging habitat; and minimizing disturbances from human activities in nest territories, at communal roosts, and at important feeding sites (MBEWG 1991). The Montana Bald Eagle Management Plan (MBEWG 1994) directs management of this species in the state. Specific objectives identified in the plan include: a minimum of 800 nesting pairs in the 7-state Recovery Area; nesting success rate of 65% in occupied sites over a 5-year period with annual average production of 1.0 fledged young per pair; population goals recognized in at least 80% of management zones with nesting potential; and continued population increases for 5 consecutive years. See the Habitat Management Guide for Bald Eagles in Northwestern Montana (MBEWG 1991) and the Montana Bald Eagle Management Plan (MBEWG 1994) for further details on management guidelines and recovery objectives.