Exploitation Competition Influences the Use of Foraging Sites by Tits: Experimental Evidence

In coniferous forests of central Sweden, tits (Paridae) and the Goldcrest, Regulus regulus, exploit nonrenewable resources in their group territories during winter. The smaller and socially subordinate species, the Goldcrest and the Coal Tit, Parus ater, forage on the outermost tree parts, while the larger and dominant Willow Tit, Parus montanus, and Crested Tit, Parus cristatus, forage on the inner tree parts. We removed Coal Tits and Goldcrests in three flocks in early winter to see if their absence would cause changes in the foraging patterns of the two dominant species. In late winter, Crested Tits foraged farther outward on branches of spruce in experimental flocks than they did in the control flocks. In spruce, Willow Tits foraged nearer the trunk than Crested Tits, and they did not respond to the experiment. In pine, Willow Tits, however, did move from branches to twigs in the absence of Coal Tits and Goldcrests. The experiment indicates that exploitation competition directly based on food depletion, without any interference, may influence the use of foraging sites by tits in coniferous forests.