Processes that structure bird communities can be divided into two general categories: niche-based and dispersal-based. By examining the proportion of coniferous specialist individuals and the species composition in two forest bird communities, we tested the relative importance of habitat preference and availability (niche-based process) versus random settlement of individuals (dispersal-based process) in determining community composition. We examined two sites on Cape Breton Island, Nova Scotia, Canada: one undisturbed, the other with a history of human disturbance. To test for random settlement, for each forest type we generated model communities based on a random selection of individuals, weighted by the relative proportions of individuals and species in the observed community. In both communities, the proportion of individuals that were coniferous specialists deviated from random. In the disturbed forest, the proportion of coniferous specialist species increased as coniferous habitat increased, consistent with an effect of habitat preference and availability on community structure. This effect was not evident in the undisturbed forest. The proportion of coniferous specialist species, especially at the disturbed site, was similar to the proportion produced by our random model, a different result than for individuals. Examining results only at the species level may mask the processes operating at the level of individuals. Total species richness at both sites was accurately predicted by our random model, suggesting that species richness may be independent of the processes that determine community composition.
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Vol. 12 • No. 4