Breeding and Feeding Ecology of Bald Eagles in the Apostle Island National Lakeshore
File(s)
Date
1986-12Author
Kozie, Karin Dana
Publisher
University of Wisconsin-Stevens Point, College of Natural Resources
Metadata
Show full item recordAbstract
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.