WINTER SEVERITY AFFECTS MIGRATION AND CONTAMINANT ACCUMULATION IN NORTHERN GREAT LAKES HERRING GULLS

Herring Gull band returns were analyzed from 1962 to 1995. Winter severity was found to affect the winter distribution of Herring Gulls banded as flightless chicks on colonies in northern Lake Huron and northern Lake Michigan. The effects of winter severity were most pronounced in adults, followed by immatures. Little response was observed in juveniles. In adults, the proportion of band returns from southerly locations increased in response to increased winter severity. In general, concentrations of organochlorine contaminants in Herring Gull eggs collected annually from northern Great Lakes colonies increased with winter severity. However, the significance of the relationship between winter severity and organochlorine concentration was compound and colony specific. This probably reflected intercolony differences in winter migration patterns with concomitant differences in contaminant exposure during the overwintering period. With increased winter severity, gulls migrated to more contaminated southerly Great Lakes locations. After spending the winter in these areas the birds returned to their breeding colonies, transferring contaminants to their eggs. The degree to which Herring Gull eggs laid on colonies located in the northern Great Lakes reflect local contaminant bioavailability will be affected by winter severity and overwinter migration patterns.