Only dominant individuals have unrestricted access to contested resources in group-living animals. In birds, subordinates may respond to intragroup contests by acquiring extra body reserves to avoid periods of food shortage, but higher body mass reduces agility and increases predation and mortality risk. Birds often live in hierarchically organized mixed-species groups, in which heterospecific individuals are considered to substitute for conspecifics as protection against predators at a significantly reduced competition cost. Crested tits (Lophophanes cristatus) and willow tits (Poecile montanus) usually form mixed-species groups during the non-reproductive season, exhibiting a nearly linear dominance hierarchy (‘despotic’ social structure) in which the highest ranking male willow tit is fourth in the overall hierarchy (above the lowest ranking crested tit). However, some groups have been observed to form a more ‘egalitarian’ dominance structure in which the adult willow tits rank second.
Results/Conclusions
A comparison of individuals in the despotic mixed-species groups revealed a strong negative correlation between subcutaneous fat stores and dominance rank in the interspecific dominance hierarchy. In egalitarian groups, willow tits had lower fat reserves and they foraged in safer parts of the canopy than in despotic groups. Egalitarian groups were also characterized by markedly less within-group aggression, more group cohesion and improved winter survival in both tit species. Nevertheless, winter survival of birds in egalitarian groups was significantly impaired in forests affected by measures of modern forestry. This suggests that more egalitarian bird societies may only be able to persist in high-quality environment.