Habitat Models for Nesting Bald Eagles in Maine

Abstract

We derived habitat models for nesting bald eagles (Haliaeetus leucocephalus) in river, lake, marine mainland, and marine island habitats. We measured 39 variables and used discriminant analysis to contrast 82 occupied nest sites and 88 random sites in Maine. Compared to random sites, bald eagle nests were located on river stretches with a larger basin area, less forest edge, and closer to the shore. The lake model indicated that bald eagle nests were positively associated with the number of superdominant trees and negatively associated with distance to water, area of land disturbed by humans, and area of land harvested for timber. The number of diadromous fish species and area of water <1.8 m deep at low tide were positively associated with bald eagle nests in marine mainland habitats; length of roads near nests was a negative variable. Nests were located on smaller marine islands than were random points (P < 0.005), resulting in nesting islands characterized by larger terrestrial and aquatic openings in the forest canopy, less forest edge, and a smaller area of combined shallow water and intertidal area. Correct classification rates for models varied from 75% for the marine island model to 100% for the river model. Similar classification rates, which never differed by more than 8%, occurred during cross validation. Validity of marine models was further suggested by the establishment of 4 bald eagle nests 51.9 km from random sites classified as nest sites; no nests have been established ?1.9 km from random sites classified as unused.