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The reproductive performance of Song Sparrows on islands of varying degrees of isolation off the southwest coast of British Columbia were explored over a period of four breeding seasons. Poor reproduction of Song Sparrows were caused by high population density, insufficient food supply, nest predation and brood parasitism by cowbirds low survival. Estimates were caused by deaths of adults and lack of breeding dipersal between years.

Many ecologists have searched for species that contribute strongly to the structure and composition of communities of organisms. It has been speculated that the Brown-headed Cowbird (Molothrus ater), a generalist brood parasite, is capable of changing songbird communities. Cowbird parasitism may reduce numbers of suitable hosts, i.e., songbirds that accept cowbird eggs and raise cowbird young. In contrast, songbird species that eject cowbird eggs, nest in cavities, feed their nestlings mainly seeds or fruits, or are too big to parasitize, should escape the effects of cowbird parasitism. Thus, cowbirds may change the composition of songbird communities by selectively depressing numbers of suitable host individuals and species. We tested this hypothesis using an existing cowbird removal program in the state of Michigan, USA. This program was designed to protect the endangered Kirtland's Warbler (Dendroica kirtlandii) from cowbird parasitism throughout its 19 200-[km.sup.2] breeding range. We compared songbird community composition in stands of young jack pine (Pinus banksiana) where cowbirds had been removed for 5-11 yr to communities in Control sites 5-10 km from cowbird traps and [is greater than]10 km from cowbird traps. We predicted that cowbird Removal sites would support greater songbird richness and evenness and a greater proportion of suitable host vs. unsuitable host individuals relative to Control sites. Results from songbird point counts revealed that species richness and evenness were very similar at cowbird Removal and Control sites and that Removal sites contained only 4.0-8.7% more suitable host individuals than Control sites. Our results suggest that cowbirds do not strongly influence the composition of songbird communities in jack pine forests of Michigan. Several factors may explain a lack of community-wide response to long-term cowbird removal, including the population dynamics of songbirds on a broader scale. We conclude that there is little support for adding cowbirds to the short list of species that can regulate entire communities. Our results have implications for the increasingly widespread use of cowbird removal as a management tool. Key words: acceptors; brood parasite; Brown-headed Cowbird; community composition; cowbird removal; demography; Dendroica kirtlandii; Kirtland's Warbler; Molothrus ater; rejecters; songbirds; species diversity.