The influence of perch tree distribution and abundance on bald eagle distribution on the northern Chesapeake Bay, Maryland
Forested shoreline is important perching habitat for bald eagles (Haliaeetus leucocephalus). Bald eagles hunt, feed and loaf on shoreline perches. I measured trees suitable for bald eagle perches along the northern Chesapeake Bay during 1990-1991 to determine the influence of shoreline perch tree availability on bald eagle distribution. The shoreline was divided into 250 x 50 m segments. A segment was considered used if at least 1 eagle had perched on it during 1985-1992. I determined the number of suitable shoreline perch trees, percent forest cover, and distance from the water to the nearest suitable perch tree for each segment.
Segments along the Chesapeake Bay had an average of 1 suitable perch tree per 10 m of shoreline. Shoreline segments used by eagles had more suitable perch trees (P = 0.0008) and a larger percent of forest cover (P = 0.0008). Suitable trees on segments with eagle use were closer to water than suitable trees on segments without eagle use (P = 0.0087). The differences in segments with and without eagle use appear to be largely due to the lack of trees in marshes which were used only seldomly. Marsh had few suitable perch trees, less forest cover and a greater mean distance from water to the nearest suitable perch tree than the other land types (P < 0.0001). These factors are unfavorable for foraging eagles and most marsh segments (66.7%) were unused, probably for this reason.
The number of suitable perch trees and the percent of forest cover were lower on developed areas than undeveloped, forested areas (P < 0.01 for both tests). Also the distance from water to the nearest suitable perch tree was greater on developed land than forested land (P < 0.01). Thus, development appears to decrease the availability of suitable shoreline perch trees when compared to forested areas.
Logistic regression models were created to predict the probability of eagle use, given the conditions at the time of this study. Varying values of development density, percent forest cover, number of suitable perch trees and distance from water to the nearest suitable tree were inputs used in these models to create curves to predict eagle use under different conditions. These curves indicated that, for a given development density, the probability of eagle use increased as the number of suitable perch trees or percent forest cover on the segment increased. Also, for a given development density, the probably of eagle use increases as the distance to water decreases.