bl_eagle

English

Bald Eagle
Haliaeetus leucocephalus
This information was scanned from The Birds of British Columbia (Campbell et al.), Volume II, pages 14-21. Volumes I, II and III of The Birds of British Columbia can be ordered electronically at: orders@ubcpress.ubc.ca from UBC Press in Vancouver, British Columbia.

Range
Bald Eagle breeds from northwestern Alaska and central Canada south to the southern United States and Baja California, but very locally distributed in interior North America. Winters from Alaska and southern Canada south along the coast and major river systems to at least the southern limits of the breeding range. Formerly bred throughout most of North America north of Mexico.

Status
Uncommon to fairly common resident along the coast of Vancouver Island, the Queen Charlotte Islands, and the adjacent mainland; seasonally and locally very common to abundant. Rare to uncommon summer visitant in the northern interior, and uncommon resident in the southern interior; local in winter in the vicinity of ice-free rivers and lakes. Widespread breeder.

Status Change
No change.

Nonbreeding
The Bald Eagle is widely distributed throughout the province. It is primarily associated with aquatic habitats including seashores, lakes, rivers, sloughs, and marshes, although it has been found in almost all habitats from sea level to 2,380 m elevation. Habitat preference varies considerably with season.

In spring, migrants follow sea coasts, rivers, and valleys northward. Concentrations are evenly distributed along the coast depending on available food resources. In the interior it is not unusual to find large groups of eagles sitting on frozen lakes awaiting the thaw or gathering near livestock calving grounds. Some frequent garbage dumps. The spring movement begins in mid-to-late February but occurs mainly in March. Adults have been seen back at nests in Victoria by 7 February, in Lumby by 22 February, in Williams Lake by 29 February, in Prince George by 14 March, in Fort St. John by 24 February, and in Atlin by 1 April.

In summer, aggregations occur only along the coast where "herring balls" and surface-feeding fishes attract both breeding and nonbreeding eagles.

The autumn movement is somewhat protracted, and appears to be correlated with the commencement of salmon spawning both on the coast and in the interior. Movements in September and early October are poorly known. On the coast, salmon spawning begins in late August and September, but eagles move into estuaries and rivers slightly later as spent salmon carcasses become available. Peaks vary, depending on location and spawning species, and may occur from mid-December (Qualicum River) to late January (Squamish River) and early February (Baynes Sound). In the interior, winter numbers and locations depend on open water and populations of waterbirds, especially American Coots. Bald Eagles are also attracted to large mammal carcasses (e.g. elk, deer, moose) killed along highways and railway tracks (Hatter 1983). Garbage dumps are used infrequently in winter.

The centre of winter abundance is along the coast. For example, 4,552 eagles were counted from various coastal sites during the mid-winter Bald Eagle survey in 1987 (Parr and Dunbar 1987). Of the birds classified, 35% were immatures, identical to results obtained on the 1986 count and similar to those reported by Millsap (1986) for 1981 and 1982 counts in the conterminous United States.

Breeding

Nests

Eggs

Young

The Bald Eagle breeds throughout the province but is only a local breeder in the Boreal Forest regions north of 56°N latitude. The centre of abundance is along the coast, where dense populations are found in the Queen Charlotte Islands and Gulf Islands. An estimate of 9,078+2,024 adult eagles was reported by Hodges et al. (1983) following an aerial survey of coastal British Columbia from 24 April to 3 May 1980.

Bald Eagles breed primarily in coniferous forests—but also in deciduous and mixed woodlands—near seashores, lakes, large rivers, and marshes, and on islands. Large trees are important habitat components. Most nest sites have an unobstructed view of the surrounding area, and a food source is usually nearby. Along the coast, most nests are within 100 m of the shore. This is also generally true for the interior, but some nests are found 1 km from water. Nests have been found from near sea level to 1,370 m elevation.

Nests:

On the coast, most nests (65%; n=543) were near the seashore, on islands, in estuaries, and at the mouths of rivers and creeks. Other nest sites on the coast included lakeshores, marshes, sloughs, lagoons, and rivers. In the interior, nests were found along lakeshores (58%; n=87), on islands in rivers and lakes (22%), and on river banks (16%) also being important. Other nests were located at the mouths of creeks, along railway tracks, and on hillsides.

On the coast, 95% of 511 nests were located in living or dead coniferous trees, including Sitka spruce (74%), Douglas-fir (19%), western red-cedar, western hemlock, and lodgepole pine. Deciduous trees (n=26) included black cottonwood (24 nests), red alder, and willow. In the interior, most nests (67%; n=73) were located in deciduous trees, including black cottonwood (49%), trembling aspen (29%), and balsam poplar. Douglas-fir (21 nests), ponderosa pine (5), and spruces were the coniferous trees used. Nests were more often in living than in dead trees. In addition, 4 nests were located on the ground atop rocky pinnacles, and 1 nest was on a cliff ledge.

Nests were usually placed in tree crotches, on branches next to the trunk, occasionally on branches a short distance from the trunk, or near the crown of the tree. Occasionally old osprey nests were used. In treeless areas, such as Triangle Island, nests were often built on the top of steep crags or on knife-edge ridges. Although the range of nest heights varied between coastal areas (ground level to 91 m) and inland areas (6 to 38 m), the average range for both areas (50% and 62% of heights respectively) was 12 to 18 m.

Nests were usually massive, cup-shaped platforms with fairly flat tops, and were constructed of dead sticks, branches, and twigs. Some pieces were up to 9 cm in diameter. Nests were lined with grasses, mosses, strips of cedar bark, leaves, and pieces of fresh conifer boughs. Some coastal nests were lined with an assortment of beach debris, including driftwood, bull kelp, eelgrass, cow-parsnip, nylon rope, dry grass, plastic tarpaulin, feathers, sea-lion bones, and assorted seaweeds. Outside diameters of 66 nests ranged from 0.5 to 3.6 m, with 58% recorded between 0.8 and 1.4 m. Outside depths (heights) of 66 nests ranged from 0.3 to 6 m with 53% recorded between 0.4 and 0.9 m. The largest nest was 6 m deep.

Eggs:

Dates for 118 clutches ranged from 12 February to 27 June, with 50% recorded between 20 April and 12 May. All eggs collected after late June were addled. Calculated dates indicate that eggs could be found as early as 7 February. Clutch size ranged from 1 to 3 eggs (1E-37, 2E-74, 3E-7), with 63% having 2 eggs. Incubation period is 34 to 35 days (Herrick 1932).

Young:

Dates for 425 broods ranged from 30 March to 1 September, with 52% recorded between 22 June and 12 July. It is rare to find young (flightless) in the nest in April and after mid-August. Sizes for 425 broods ranged from 1 to 3 young (1Y-253, 2Y-159, 3Y-13), with 60% having 1 young. Fledging period is 10 or 11 weeks (Harrison, C. 1978).

Remarks
Gerrard (1983) suggests that Alaska and British Columbia account for 48,000 (69%) of the 70,000 Bald Eagles estimated to occur in North America. Populations in western North America are generally stable.

It has been suggested by many biologists, including ServUeen and English (1979), that there are regular migratory movements of Bald Eagles along the Pacific coast and this is supported by our data. Recently, through radio-tagging, Hodges et al. (1987) have shown that some of the eagles found wintering along the British Columbia coast are from Alaska. Their studies indicate that 73% of first-year and immature eagles disperse through the winter season into British Columbia (and farther south) while 27% remain in southeast Alaska during the summer. They also suggest that immature birds that overwinter in British Columbia may remain there for several years.

The salmon spawning rivers and adjacent forests are undoubtedly important to wintering Bald Eagle populations in British Columbia. The degree of importance has not been determined but can be inferred from studies carried out in Washington State.

Stalmaster and Gessaman (1984) note that winter food shortages could naturally limit eagle numbers, and associated food stress could also lower the birds' reproductive performance after they leave the wintering grounds. Since winter densities of raptor populations are thought to be limited by food, protective management policies that reduce energy stress could reduce overwinter mortality. One method of reducing stress would be to ensure that some mature coniferous stands remain near the salmon streams. Anthony, R.G. et al. (1982) note that eagles often select mature and old-growth coniferous forest for roosts. Stalmaster and Gessaman (1984) demonstrate the importance of these forests to the Bald Eagle. In their study on the Nooksack River, they found that the eagles spend 67.5% of their day in roosts, and most of the eagles roosted in coniferous rather than deciduous forests even though the eagles had to expend more energy to travel there. By roosting in conifers, where temperatures and long-wave radiation are higher, wind velocity is low, and rainfall is considerably reduced, the eagles save about 5% of their daily energy budget even after accounting for the energy costs of flying to and from the roost. See Farr (1987) for a description of Bald Eagle roosts in the Fraser Lowlands.

Another stress on wintering eagles is human disturbance—the leading cause of eagle mortality in North America (Newtcin 1979). Again on the Nooksack River, Stalmaster and Newman (1978) found that the distribution of eagles there reflected the effect of human activity and had a significant influence on their feeding behaviour. Disturbed birds did not return to the same feeding area until several hours after the disturbance occurred, and only when the disturbance had ended. They also found that activities directly on the river, such as boating and fishing, were most disturbing if activities did not regularly occur there. Stalmaster and Newman (1978) suggest that activity restriction zones be created for Bald Eagle wintering grounds, preferably in conjunction with vegetation buffer zones. They note that boundaries of 250 m would be sufficient to protect 90% or more of the population, as most of the eagle avoidance flights were less than that distance.

Steenhof and Brown (1978) list guidelines for preserving and enhancing wintering sites for Bald Eagles.

A very useful bibliography has been published by the National Wildlife Federation (Lincer et al. 1979).

POSTSCRIPT: The mid-winter Bald Eagle survey in 1988 tallied 8,400 birds of which nearly three quarters were in the southern coastal area (Farr and Dunbar 1988). The percentage of immatures was 37, slightly higher than 1986 and 1987 values. A preliminary estimate of the mid-winter population for the province is between 20,000 and 30,000 birds.