Community-wide impacts of a generalist brood parasite, the Brown-headed Cowbird (Molothrus ater)

Many ecologists have searched for species that contribute strongly to the s tructure and composition of communities of organisms. It has been speculate d that the Brown-headed Cowbird (Molothrus ater), a generalist brood parasi te, is capable of changing songbird communities. Cowbird parasitism may red uce 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 Michi gan, USA. This program was designed to protect the endangered Kirtland's Wa rbler (Dendroica kirtlandii) from cowbird parasitism throughout its 19 200- km(2) breeding range. We compared songbird community composition in stands of young jack pine (Pinus bnnksiana) where cowbirds had been removed for 5- 11 yr to communities in Control sites 5-10 km from cowbird traps and >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 fr om songbird point counts revealed that species richness and evenness were v ery similar at cowbird Removal and Control sites and that Removal sites con tained 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, in cluding the population dynamics of songbirds on a broader scale. We conclud e that there is little support for adding cowbirds to the short list of spe cies that can regulate entire communities. Our results have implications fo r the increasingly widespread use of cowbird removal as a management tool.