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.在整个西方世界，大量民众会定期为野生鸟类投喂食物。然而，已有研究证实冬季投喂会对鸟类后续繁殖产生负面影响，这凸显出我们需要加深对调控野生鸟类响应补充食物的内在机制的理解。本研究旨在验证如下假设：补充投喂的携带效应（carry-over effect）可通过影响繁殖鸟类的表型来介导，具体途径包括通过改变冬季存活率来改变种群的表型结构，或是直接影响繁殖鸟类的身体状态。本研究以青山雀（Cyanistes caeruleus）为研究对象，开展了为期三年的景观尺度野外调查，证实了补充食物的营养组成是决定携带效应产生何种结果的关键因素。研究结果显示，相较于投喂高脂食物组与未投喂组，在前一个冬季接触过富含维生素E食物的繁殖种群，其个体的羽毛类胡萝卜素（carotenoid）浓度更低，这表明该种群在投喂前的表型状态更差。这表明冬季补充投喂可改变繁殖季种群的表型结构，其潜在机制可能是提升了低质量个体的存活率与种群招募率。但冬季投喂高脂食物会对繁殖鸟类的氧化状态产生不利影响，而这类表型差异最终会影响其繁殖成功率。本研究结果揭示了补充投喂影响野生鸟类种群的复杂机制。