Long-Term Studies of Common Gulls (Larus Canus) in Estonia: Responses to Environmental Conditions

Abstract

The population of Common Gulls has been studied since 1962, making the analysis of longterm changes in the population and the impact of the changing environment possible. Due to the fixed clutch size, the reproductive lifespan and survival of offspring from hatching to maturity, both affected mainly by winter severity, are the prime creators of the variation in lifetime reproductive success (fitness). We point out that climatic conditions influence both the annual breeding success (hence, fitness) and the age at first breeding through the laying date (early years, late years). However, over the 35 year-long period of studies we have not found any persistent advancement in the laying date. We emphasise that the use of phenotype values in the analysis of natural selection may lead to incorrect results due to phenotypic plasticity. The invasion of Herring Gulls in the middle of the 1970s has caused a 1.3-fold and 3.5-fold increase in natal fidelity of males and females, respectively, resulting in the possibl...