Long-term estimates of adult survival rates of urban Herring Gulls Larus argentatus and Lesser Black-backed Gulls Larus fuscus

Abstract

Urban gull populations have increased dramatically in the last 40 years, leading to widespread concerns about potential nuisance to humans, but little accompanying research into their ecology. This study aimed to provide the first long-term estimates of apparent adult survival rates for urban Herring Gulls Larus argentatus and Lesser Black-backed Gulls L. fuscus, based on colour ringing in Bristol, southwest England. Resightings of adult birds covering 18 years (1990–2007) were analysed using capture–mark–recapture methods, with candidate models testing for differences in survival and/or resighting rate through time and between the sexes. Both species showed high apparent annual survival rates (>0.90) in the early 1990s that declined to <0.70 by 2007. Male survival rates were higher than female rates in Lesser Black-backed Gulls, and male resighting rates were higher in both species. In the early 1990s, Bristol's urban gulls displayed similar adult survival rates to published estimates for rural colonies. Both species showed evidence of long-term declines in apparent survival, which may either reflect actual reductions in survival or increased permanent emigration from the Bristol colony. Anecdotal evidence supports the latter, linking emigration to urban redevelopment or human intervention.