Effects of body mass on the foraging behaviour of subordinate Coal Tits Parus ater

Several mechanisms can explain individual differences in foraging behaviour , such as variation in predation risk between patches, variation in the abi lity of individuals to detect or escape from predators, variation between i ndividuals in their requirement for food, the quality and abundance of food in different patches, phenotypic variation giving rise to differences in r esource use (exploitation hypothesis) and interference competition such as the exclusion of subordinate individuals by dominants. Subordinates can dev elop compensation mechanisms. One of these mechanisms is morphological diff erentiation. However not every change in morphology can account for the sam e variation in behaviour, since some morphologies can be ecologically more plastic than others (i.e, some morphs can exploit a broader niche than othe r morphs). Under controlled conditions in the Coal Tit Parus ater, we teste d whether (1) differences in resource use were explained by either the expl oitation hypothesis or by the interference hypothesis, and (2) the presumed costs of subordination can be reduced through different ecological plastic ities associated with different morphologies. Our results support the inter ference hypothesis as there are no differences in hanging behaviour between dominants and subordinates when foraging solitarily; while in the presence of other individuals, we observed differences in foraging behaviour that v aried with social status. Our results also show that body mass influenced f oraging behaviour; lighter birds can exploit patches where hanging postures are needed more easily than heavier birds. Moreover, this relationship var ied among individuals, as predicted by the ecological plasticity hypothesis . Lighter subordinate individuals used hanging postures more frequently tha n heavier ones, differentially reducing the costs of subordination. We prop ose that differences in the breadth of ecological niche due to differences in morphology can reduce the costs of subordination.