Biology of

Abstract

Late-nesting Ring-billed Gulls in this study laid fewer and smaller eggs and hatched fewer young than earlier nesting pairs. Birds nesting late in the season appeared to lack sufficient stimuli for incubation, as indicated by their increased restlessness on the nest and higher desertion rates. Factors affecting their behavior include physical condition and social and environmental stimuli. Late-nesting gulls had a lower body weight and condition index than peak-nesting gulls. Their smaller egg size was attributable to decreased amounts of albumen. Analyses of blood metabolites and hormones were inconclusive. It is suggested that the low reproductive success experienced by "late"-nesting Ring-billed Gulls in this study results predominantly from factors during the incubation period.