Wind stress affects foraging site competition between Crested Tits and Willow Tits

Crested Parus cristatus and Willow tits P. montanus are severe food competitors owing to their similar body size, diet and foraging sites. In Belgium, they occupy overlapping territories and form mixed-species winter hocks. Earlier, we found environmental stress to govern foraging strategies of first year Crested Tits (subordinates) in unispecific flocks. I here test whether niche shifts of Willow Tits (subordinates) in mixed flocks with Crested Tits (dominants) are in the direction expected if interspecific competition governs foraging site segregation when wind velocity increases. At high wind velocity, Willow Tits shifted to sites higher up and further out in trees in the presence, but downward and inward in the absence of Crested Tits. In the latter case, they used similar positions to allopatric Willow Tits. At high wind velocity, foraging shifts of Willow Tits decreased the niche overlap with Crested Tits foraging in the same tree, but there were no such shifts when foraging in neighbouring ones. The fact that Willow Tits frequently join coherent flocks with Crested Tits on windy days, suggests that environmental stress might govern flocking strategies adopted by subordinate tit species in dominance-structured associations.