Breeding and Feeding Ecology of Bald Eagles in the Apostle Island National Lakeshore
Kozie, Karin Dana
University of Wisconsin-Stevens Point, College of Natural Resources
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Food habits and foraging areas, and chemical contaminants in eagle foods and carcasses were studied in bald eagles (Haliaeetus leucocephalus) nesting within the Apostle Island National Lakeshore (AINL) during 1984 and 1985. Data on food habits collected during banding operations in 1983 and 1986 are also included. Productivity was monitored during 1984-1986. Food habits were determined by collecting prey remains from below nest and perch trees and by direct observation. Fish comprised 52.4% of prey remains and 97.0% of.observations. Common species included longnose sucker (Catostomas catostomas), burbot (Lota lota), white sucker (Catostomus commersoni), and round whitefish (Prosopium cylindracem). Birds, primarily herring gulls (Larus argentatus), comprised 46.9% of prey remains and 1.5% of observations. Mammals comprised 0.7% of food remains and 1.5% of observations. Organochlorine and PCB residues were present at low levels (DDE: x = 0.07 ppm, PCB: x = 0.21 ppm) (wet weight) in fish. Adult and immature herring gulls contained higher concentrations (DDE: x = 5.4 ppm, PCB: x = 13.5 ppm) and appear to be the major source of elevated contaminant levels in the Apostle Island bald eagle population. The source of herring gull contamination is unknown. Migratory birds, wintering in areas where DDT is still in use, may add additional contaminants to the eagle food chain. Eagle feeding areas were located primarily along the shores near nest areas during late incubation, brood rearing and early postfledging periods. An average of 0.9 young/occupied nest were produced in the Apostle Islands during 1984-1986 compared to an average of 1.3 young/occupied nest produced statewide in Wisconsin during those years. Nestling mortality was 27% among island nests (4 of 15 young). Contaminant levels in nestling bald eagle carcasses collected from Lake Superior nests were higher than those collected inland, suggesting local contamination. Replacement of adult females in 2 nests and the known death of an adult female at a 3rd nest, indicates a high turnover among breeding adults in the AINL during 1985. Current production in the Apostle Islands may be due to a younger breeding cohort, with low contaminant levels, dispersing from the inland population. Disturbance did not appear to be a factor affecting productivity or survival.