Non-competitive habitat use by foraging passerine birds during spring migrations

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Abstract
We studied the seasonal occurrence and the habitat of migratory passerines, in order to test the effects of the changes in the density of possible competitors on the foraging niches of the passerines. Intra-specific competition affected the niche of two species out of nine. Their niche breadths increased significantly with increasing density of the species, as theoretically expected. However, no inter-specific competitive effects were detected. Niche overlaps were not correlated with the densities of possible competitors, and the seasonal occurrence of a species through the migration season was not correlated with the presence of other species with similar habitat preferences. Migrant species appeared to exploit foraging resources opportunistically and they did not use any mechanism of segregation from residents.