Graduate Thesis Or Dissertation

Environmental contaminants and ecology of bald eagles in southcentral Oregon

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  • Food habits and levels of organochlorine compounds, lead, and mercury in resident and wintering bald eagles were studied in southcentral Oregon and California, 1979-83. Food habits were assessed by examination of castings from winter communal roosts, identification of 2938 prey items found at nest sites and foraging areas, and observations of 16 eagles with radios. Contaminant residues in prey were determined by analyses of 290 pooled samples of potential prey. Contaminant residues in eagles were determined by analyses of 13 addled eggs, blood samples from 24 adults, 8 sub-adults, and 82 nestlings, and carcasses of 11 eagles. Non-resident eagles concentrated in the southern Klamath Basin during winter months and fed largely on microtine rodents and cholera-killed dabbling ducks and geese. Contaminant residues in samples of prey from the wintering area were low with the possible exception of lead shot in waterfowl, which presented a potential for lead poisoning in eagles. Wintering bald eagles did not have elevated body burdens of organochlorines which have been associated with reproductive problems. Diets of resident eagles in southcentral Oregon were highly diverse, changed seasonally, and differed markedly by geographic region. Eagles fed largely on fish during summer months with the importance of ducks and fish-eating birds increasing during the fall and late winter. Most prey of resident eagles were taken live or pirated from osprey; scavenging comprised less than 20% of the observed predation. Contamination of the majority of the prey of resident bald eagles was fairly low. However, DDE, PCBs, and mercury were consistently detected at moderate levels and indicated biomagnification in the food-chain. Fish-eating birds in the eagles' diets were apparently the source of elevated environmental contaminants in the eagles on Upper Klamath Lake. Concentrations of DDE in eagle blood and eggs indicated that reproductive success of specific nest sites in the Klamath Basin was reduced. Although contamination was not at levels associated with critical population declines, its effects should not be discounted considering the number and severity of other factors impinging on eagle populations.
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