Bald Eagle Habitat Model
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Draft Date:
August 2002

Species:
Bald Eagle, Haliaeetus leucocephalus

Use of Study Area Resources:
Reproduction and wintering. Bald eagles occur throughout North America, from Alaska to Mexico. They typically winter near large ice-free water bodies (Steenhof 1978). Within the study area they breed in Maine and northeast New Hampshire. Sites in Massachusetts, coastal Maine, and New Hampshire offer wintering habitat.

Habitat Requirements:
Nesting. Eagles select areas with low human disturbance, suitable forest structure, and abundant prey. Because fish are important prey, nests are nearly always associated with fishable waters. Livingston et al. (1990) applied discriminant function analysis to nest site characteristics to produce a model of bald eagle habitat. River nest sites typically were close to the shores of rivers with large aquatic areas and little forest edge. Lake nest sites were near to water, had superdominant trees, and little overall human disturbance. They were also characterized by having a number of warmwater fish species, and relatively large surface area (> 30 ha). Marine mainland nesting habitats were associated with a high diversity of diadromous fish, large shallow water areas at low tide (<1.8 m deep), and few roads. Marine island nesting habitat differed from the other habitat types in that waterfowl and seabirds were the primary prey (see also Todd et al. 1982), and smaller islands were preferentially selected (Livingston et al. 1990).  

Foraging. Todd et al. (1982) found that Maine eagles at inland sites fed primarily on suckers, pickerel, and bullheads. Anadromous fishes were important at mainland coastal sites, while black ducks, herring gulls and other colonially nesting birds were the most common prey of eagles at island sites.

Livingston et al. (1990) calculated a typical foraging range as within 1.5 km from nest sites, based on half the spacing between nests in areas heavily used by eagles. This was supported by McCollough's observation (1986 in Livingston et al.1990) that more than 90% of fledgling eagles perched within 1.5 km of their natal nests (n=13), suggesting that suitable foraging resources were within that distance.

Wintering. In winter, eagles aggregate at areas having ice-free waters, seclusion from human activity, large trees with stout, easily accessible branches, and protection from strong winds (Southern 1963, Steenhof 1978, Hansen 1987). Wintering eagles in Maine feed along open coastal waters, preying on waterfowl and scavenging carcasses (Todd et al. 1982). Coastal and inland sites are used in New Hampshire and Massachusetts (Deluca 1993, Cook et al. 1995).

Sensitivity to Disturbance

Tolerance of disturbance varies among individual eagles and with season; 500 m is approximately the closest that humans (Fraser et al. 1985) or other eagles (Mahaffy and Frenzel 1987) can approach a nesting eagle before disturbing it. This distance is typically converted to a one-quarter mile radius for management purposes.

Functional nesting habitat is generally considered to encompass a minimum of 640 acres, including foraging and nesting habitat, and should be contiguous acreage unless the habitat elements are known to be separate (USFWS 1983). Nesting habitat management guidelines prohibit most activities within a primary management zone of 750 to 1500 feet from the nest, and particularly prohibits unauthorized human entry during the nesting season (USFWS 1987). Activities which alter the site such as logging, land clearing, development, construction, mining and low-level aircraft should be prohibited within a radius of 1 mile (1.6 km) of nest sites (USFWS 1987).

Mapping and Modeling:
Comprehensive surveys by New Hampshire, Massachusetts and Maine wildlife programs, and New Hampshire Audubon Society, have resulted in fairly complete information on eagle nesting locations. We obtained major wintering areas from narrative information provided by New Hampshire (Deluca 1993, Cook et al. 1995) and Massachusetts. We buffered the nest and winter roost sites at 0.25 mile.  Within this buffer we re-scored appropriate cover types (see table, below) for nest sites 1.0 and wintering sites 0.9. The difference in score was used more to enable the sites to be distinguished on a map than to indicate a difference in biological importance.

Foraging habitat was mapped within 1.5 km of the known nesting or wintering areas (a radius of about 2 km centered on the nest or winter roost points). Based on Livingston et al. (1990), we selected riverine, lake, and shallow coastal waters (detailed below) within this distance and scored these 0.8.

Supplementary foraging and nesting habitats were modeled, using Livingston et al. (1990) findings.  In general, the modeling consisted of identifying feeding areas, then identifying adjacent nesting areas.  Four models were developed: riverine, lake, coastal, and island.

Rivers with large aquatic areas were identified by selecting NWI "double line" riparian polygon features (see table, below).  Modeled riverine foraging areas were scored 0.2. Suitability of adjacent forested areas for nesting then was based on distance from the river (optimal if within 0.5 km, suitable if within 1 km) and on the extent of edge between forest and non-forest cover types (relatively limited edge was regarded as optimal, otherwise the cover was regarded as just suitable. Modeled riverine nesting habitat scores were the product of these factors, and had resulting values of 0.1, 0.3, or 0.5.

All lakes were selected which were over 30 ha in surface area (we lacked comprehensive information on warmwater fisheries) and scored 0.2.  Suitability of adjacent areas for nesting then was based on distance from the lake (as for rivers, above), and distance of forest types from development/agriculture (< 90 m = unsuitable, otherwise < 500 m = suitable, >= 500 m = optimal).  Modeled lake nesting habitats were scored had values of 0.1, 0.3, or 0.5, based on these factors.     

Coastal feeding habitats were mapped by selecting areas that were mid-intertidal down to -6' mean low water in depth, and scoring these according to the coincidental occurrence of three diadromous fish species (American eel, alewife, blueback herring).  Where 2 or 3 species occurred in shallow enough water, the foraging habitat was considered optimal (scored 0.4).  Where one species occurred the habitat was scored as suitable (scored 0.2).  Nesting habitat was mapped as suitable forest types within 1 km of foraging habitat and having a road density of about 20% of the coastwide maximum, largely agreeing with the observed distribution of eagle nests.  Modeled coastal nesting habitats were scored 0.5.

Marine island nesting habitat was not specifically modeled independently of other coastal habitat because Livingston et al. (1990) noted their island model was less reliable than their river, lake, and coastal mainland models.  Also, our coastal model (above) identified habitat on 145 of 151 (96%) islands with nests, and on only 648 of 1176 (55%) islands without eagle nests.  Therefore, we used our coastal model for mainland coastal and marine island habitats.

Nominal Suitability Values for Foraging, Nesting or Wintering Sites

Overall habitat suitability was the maximum score for nesting/winter roosting or foraging.

Model testing: The bald eagle nest and winter roost areas had been used to modify scores of habitat mapped from cover types, not to identify habitat, and so they remained available for testing the overall habitat map. We compared the presence of habitat near a random set of 798 upland points to that for locations at which bald eagles were observed. Of the 353 sites with birds, 345 had mapped habitat, while 343 sites out of the 798 randomly distributed sites had habitat. Birds occurred in areas mapped as having habitat more frequently than would be expected by chance (Chi-square highly significant).

Sources:
Cook, R.A., C.J. Martin, D. DeLuca and L.S. Deming. 1995. New Hampshire endangered species program status and management report 1 April 1994 - 31 March 1995 Project No. EW-1-13. Unpublished report to USFWS.

DeLuca, D. 1993. Wintering eagles on Great Bay. New Hampshire Fish and Game Dept. 25 pp.

Fraser, J.D., L.D. Frenzel and J.E. Mathisen. 1985. The impact of human activities on breeding bald eagles in north-central Minnesota. J. Wildl. Manage. 49:585-592.

Hansen, A.J. 1987. Regulation of bald eagle reproductive rates in southeast Alaska. Ecology 68:1387-1392.

Livingston, S.A., C.S. Todd, W.B. Krohn and R.B. Owen, Jr. 1990. Habitat models for nesting bald eagles in Maine. J. Wildl. Manage. 54(4):644-653.

Mahaffy, M.S. and L.D. Frenzel. 1987. Elicited territorial responses of northern bald eagles near active nests. J. Wildl. Manage. 51:551-554.

Southern, W.E. 1963. Winter populations, behavior, and seasonal dispersal of bald eagles in northwestern Illinois. Wilson Bull. 75(1):42-55.

Steenhof, K. 1978. Management of Wintering Bald Eagles. USFWS Biological Services Program. FWS/OBS-78/79. 59 pp.

Todd, C.S., L.S. Young, R.B. Owen, Jr. and F.J. Gramlich. 1982. Food habits of bald eagles in Maine. J. Wildl. Manage. 46:636-645.

USFWS. 1983. Northern States Bald Eagle Recovery Plan. 75 pp.

USFWS. 1987. Habitat Management Guidelines for the Bald Eagle in the Southeast Region. 9 pp.