Factors influencing the fat load variation in three wintering bird species under stable food access conditions

Abstract

Winter is one of the crucial periods of the bird’s year-round life cycle. Fat reserves in bird species are prone to a wide range of factors, with weather conditions and a bird’s social status being two major ones. In our study we analysed the variation in fat reserves in birds with stable, constant access to food resources in three of the most common wintering species: the great tit Parus major, the blue tit Cyanistes caeruleus and the greenfinch Chloris chloris. The highest mean value of fat score was found in the greenfinch, whereas in both species of tits these values were equally lower. For all three species the amount of fat reserve was correlated with the same factors. It increased together with the decrease of mean average temperature, and with the course of daytime as well as the season-site index (calculated as data collected at a given site in a particular year). Contrary to many previous surveys we did not find dependence of the fat score level on individual dominance status, which is probably linked to the abundance of food in catching sites.