Lake Ontario herring gulls (Larus argentatus) did not defend their nests in the normal manner and were less attentive (P < 0.05) than those on Kent Island, New Brunswick [Canada]. Incubating Lake Ontario gulls appeared to apply less heat to their eggs. Within Lake Ontario, a comparison of nest attentiveness and nest air temperatures between successful and unsuccessful nests revealed similar differences. The decreased nest defense of Lake Ontario gulls was sufficient to account for the high incidence of egg loss observed. The variability in nest air temperature was sufficient to increase embryonic mortality based on studies of other species. These abnormalities probably result from pollutant-induced endocrine dysfunction.