Description

Bald eagles Haliaeetus leucocephalus nesting on Green Bay, Lake Michigan, have extremely low reproductive rates, in comparison to eagles nesting in inland Wisconsin and Michigan 0.4 vs. 1.1 young per occupied territory, Colborn 1991, Bowerman 1993. We investigated two factors which can affect eagle reproductive rates: organochlorine contaminants and food availability. Organochlorine contaminants specifically DDE and total PCBs were measured in addled eggs and nestling blood samples. Food availability was assessed by measurement of a behavioral index to food availability: food delivery rate to the nestlings. Other behavioral indices that have been associated with food availability time spent feeding and adult attendance at nests were also measured and compared to reference values for inland Wisconsin eagles. Mean contaminant concentrations in addled eggs from Green Bay were 10.3 ppm wet weight DDE and 35.0 ppm wet weight total PCBs 1986 1992, n8. Mean concentrations in nestling blood plasma were 53 ppb wet weight DDE and 207 ppb wet weight total PCBs 1987 1995, n8. Indices of food availability were generally normal, in comparison to inland reference data Mean food delivery rate to the nestlings was 2.21 .25 items per nestling per day n4 nests. Time spent feeding averaged 8.5, 8.2 and 9.3 of the day in the early, mid, and late phases of the nestling period n3,4,4. Adult attendance at the nest averaged 88.9, 26.5, and 5.7 of the day in the early, mid and late phases n3,3,3. None of the behavioral measures differed significantly from inland reference values, which suggests that the prey base was adequate. We concluded that it is very likely that organochlorine contaminants caused all or most of the depression in reproductive rates of Green Bay bald eagles.

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