Body Size, Interspecific Interactions, and Use of Foraging Sites in Tits (Paridae)

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Abstract
In mixed-species flocks, the heavier tit species forage in the inner tree parts, while the smaller species exploit food items of the outer canopy. Using aviary experiments we studied the relative importance of two possible explanations for the body-size-foraging site correlation. The first hypothesis is that larger bodied tits are more efficient than smaller ones in exploiting the inner tree parts, while the reverse would be true in the outer canopy. However, the heavier species, the Willow Tit Parus montanus, was not more efficient in food searching in the inner tree parts than the smaller species, the coal tit Parus ater. The second hypothesis is based on interference competition with the socially dominant (heavier) species selecting the most profitable foraging sites, and forcing the smaller species to less rewarding places. In fact, in confrontation experiments only individuals of the smaller species changed their foraging sites in the presence of an individual of the other species in the aviary. In sum, body size influences social dominance relations and thereby relegates the smaller species to poorer foraging sites, but presumably gives them an advantage in exploitation competition via reduced energy demands for basal metabolism.