Differences in daily mass gain between subordinate species are explained by differences in ecological plasticity

Abstract

During winter, small birds in temperate zones increase their internal fat reserves to combat harsh environmental conditions and the drastic reduction in predictability of resource access. High fat reserves provide some benefits related to starvation-risk reduction, but also imply some costs related to increased predation risk. In general, subordinate species should increase their level of fat reserves in response to the unpredictability of resource access when in competition with dominants. However, the magnitude of such reserve increases could differ between subordinate species if opportunity of access to resources varies interspecifically in relation to differences in niche breadth due to differences in morphology. We tested the hypothesis that subordinate species (Parus cristatus) with decreased access to food will accumulate more fat than subordinate species (Parus ater) with greater opportunities. Results supported the prediction. We found that crested tit, the species with lower ecological plasticity and thus less access to food under competition, stores more fat daily than the coal tit, the species with higher ecological plasticity owing to its ability to forage while upside down.