Effect of Age, Colony of Origin and Year on Survival of Yellow-Legged Gulls Larus michahellis in the Bay of Biscay

Abstract

We studied sources of variation in apparent local survival of yellow-legged gulls ringed as chicks in a number of colonies with different size trends. Specifically, our aim was to test whether individuals hatched in colonies with decreasing population trends had lower survival rates than those from stable or increasing colonies. From 2006 to 2013, 3,024 chicks were colour-ringed in four colonies along the coast of the Basque Country (from east to west): Ulia, Santa Clara, Guetaria and Izaro. Sighting data of these gulls were compiled from August 2006 to June 2013. Cormack-Jolly-Seber models with mixtures were used to estimate apparent survival (hereafter, survival). Overall, survival differed between two age classes (first-year birds < older birds), colony of origin and in relation to year. The Izaro colony, one of the decreasing colonies, showed the lowest survival rate. However, survival was observed to be reasonably high in another declining colony. Hence we did not find a general link between survival and colony trends. Survival did not seem to be affected by colony size and we did not find evidence supporting a decrease in survival across the study period. More research is needed to disentangle factors explaining variation in local survival in yellow-legged gulls.