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Community-wide impacts of a generalist brood parasite, the brown-headed cowbird (molothrus ater)

University of British Columbia

Many ecologists have searched for species that contribute strongly to the structure and composition of communities of organisms. It is widely believed that the Brown-headed Cowbird, 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 have evolved egg ejection behaviour, nest in cavities, feed cowbird nestlings an unsuitable diet or are too big to parasitize, will generally escape the effects of cowbird parasitism. Thus, cowbirds may change the composition of entire songbird communities by depressing numbers of suitable host individuals. I tested this hypothesis using an existing cowbird removal program in the state of Michigan, USA . This removal program was designed to protect the endangered Kirtland's Warbler from high levels of cowbird parasitism, throughout its 19 200 km² breeding range. I compared songbird composition in stands of young jack pine where cowbirds had been removed for 5-11 years to Control sites 5-10 km from cowbird traps and Control sites >10 km from cowbird traps. I predicted that cowbird Removal sites would support greater songbird diversity and a greater proportion of suitable host vs. unsuitable host individuals relative to Control sites. Results from songbird point counts revealed that species diversity was very similar at cowbird Removal and Control sites but Removal sites contained 4.0 - 8.7 % more suitable hosts than Control sites. I conclude that cowbirds only weakly influence the composition of songbird communities in jack pine forests of Michigan. It remains to be shown that cowbirds affect songbird community composition more strongly in other areas, e.g., midwestern USA, where cowbirds are more abundant.

Thesis/Dissertation

application/pdf

2009-06-10

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http://hdl.handle.net/2429/8925

Zoology

University of British Columbia

UBCV

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