Interspecific Competition in Tits Parus spp. and the Goldcrest Regulus regulus: Foraging Shifts in Multispecific Flocks

Abstract

The foraging site shifts of tits and the goldcrest, which were caused by the presence or absence of other species in flocks during the winter in northern Finland, were studied. Great tits occur only in forests near houses, and the crested tit avoided the foraging sites of the great tit (P. major) in these forests. The willow tit (P. montanus) avoided trees used by the crested tit (P. cristatus) when both were foraging together. In the best studied case, willow tits moved to outer and upper parts of spruce in the presence of great tits, which foraged in lower and inner parts but, in the presence of crested tits, which prefered upper and outer parts of trees, willow tits moved to inner and lower sections of spruce. Using all the available comparisons in pine or spruce, species significantly avoided one another''s foraging tree parts when together in the same flock. Foraging shifts seem to involve interference competition, social subordinates being more likely to change their foraging sites than social dominants. However, exploitative competition without aggression may be involved as a supplementary factor; avoidance was often mutual and social dominants sometimes appeared to avoid the foraging sites of the subordinate species in the encounter. Results strongly suggest that interspecific competition is currently, in ecological time, affecting the foraging behavior of these coexisting birds.