Mixed-species foraging aggregations and heterospecific attraction in boreal bird communities

We investigated whether different species of birds associated with each other while foraging during the breeding season. On average 50% of foraging observations were in mixed-species foraging aggregations, but with extensive interspecific variation. The occurrence of mixed-species foraging aggregations was relatively constant from the end of May until early July, with no indication that the appearance of fledglings increased the frequency of aggregations. These mixed-species foraging aggregations consisted of a wide variety of species in terms of foraging niche: typical flycatchers, specialized arboreal insectivores, and generalists were observed foraging together. The titmice species (Parus spp.) seemed to be preferred as foraging companions. The sites where foraging aggregations were observed did not differ from an equal number of randomly chosen sites with respect to forest vegetation structure. The dispersion of individuals in study areas was analysed using a 50 m x 50 m quadrat as the sampling unit. Altogether, 96 quadrats were surveyed. The dispersion of individual birds with respect to individuals of other species was non-random, and significantly clumped within quadrats. We used two playback experiments to test whether the aggregations were due to active search for heterospecific companions. In the first experiment, 20 randomly assigned sites received two 10-min treatments: willow tit (Parus montanus) song and silence as control. In the second experiment, we had two control treatments at 30 playback sites for the willow tit sons, the song of the olive-sided flycatcher (Contopus borealis) and classical music. More species were observed during willow tit song than during any of the controls. This suggests that aggregated dispersion pattern and the existence of mixed-species foraging aggregations were due to active search for heterospecific companions, and also that recognition of neighbouring species was involved. We suggest that birds actively aggregate in mixed-species Foraging groups to gain protection against predators and/or enhance feeding efficiency. The results support the view that positive interactions have an importance in structuring northern breeding bird communities.