Effects of winter food provisioning on the phenotypes of breeding blue tits

Throughout the Western World huge numbers of people regularly supply food for wild birds. However, evidence of negative impacts of winter feeding on future reproduction has highlighted a need to improve understanding of the underlying mechanisms shaping avian responses to supplementary food. Here, we test the possibility that carry-over effects are mediated via their impact on the phenotypes of breeding birds, either by influencing the phenotypic structure of populations through changes in winter survival and/or by more direct effects on the condition of breeding birds. Using a landscape-scale three-year study of blue tits (Cyanistes caeruleus), we demonstrate the importance of nutritional composition of supplementary food in determining carry-over effect outcomes. We show that breeding populations which had access to vitamin E-rich foods during the previous winter were comprised of individuals with reduced feather carotenoid concentrations, indicative of lower pre-feeding phenotypi...