Data from: 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 phenotypic condition,
compared to fat-fed and unfed populations. This suggests that
supplementary feeding in winter can result in altered population
phenotypic structure at the time of breeding, perhaps by enhancing
survival and recruitment of lower quality individuals. However,
supplementation of a fat-rich diet during winter was detrimental to the
oxidative state of breeding birds, with these phenotypic differences
ultimately found to impact upon reproductive success. Our findings
demonstrate the complex nature by which supplementary feeding can
influence wild bird populations.