Rank-dependent fattening strategies of Willow Tit Parus montanus and Crested Tit P. cristatus mixed flock members

Abstract

I studied changes in body mass in response to the presence ofheterospecifics in mixedspecies flocks of free-living Willow and Crested Tits . Body reserves of Willow Tits as subordinate flock members were found to be dependent on the presence of dominant Crested Tits . At dusk, Willow Tits were heavier than Crested Tits . Body reserves of members of mixed-species flocks negatively correlated with dominance rank within both intraspecific and interspecific hierarchies. Removal experiments confirmed results of field observations, revealing a causal relationship between rank and ability to choose the safest fattening strategy . Body mass and fat reserves of Willow Tit alpha males decreased in the absence of Crested Tits . In contrast, body mass and fatness of dominant individuals of Crested Tits remained at the same level. To control for changes in body mass resulting from a higher rank and smaller flocks, the foraging height of remaining individuals of both species was recorded. Willow Tits responded to removal of Crested Tits by foraging closer to the top of the tree . No shifts were observed in groups with only Crested Tits . Fattening strategies dependent on social status represent a flocking cost to individuals belonging to a subordinate species. Further investigations should be done into the benefits of mixed-species flocking, which is a social behaviour common in parids .