Brown-headed Cowbird (Molothrus ater) parasitism on pewees (Contopus spp.) is uncommon or rare in North America, suggesting that pewees sympatric with cowbirds either reject cowbird eggs or are not selected by cowbirds for parasitism, or that cowbirds are unable to parasitize pewee nests. At Delta Marsh, Manitoba, we inspected Eastern Wood-Pewee (C. virens) nests to determine parasitism frequency and experimentally parasitized pewee nests with real and artificial Brown-headed Cowbird eggs to assess their response to parasitism. No parasitism was recorded at 20 unmanipulated nests or at 12 experimentally parasitized nests, whereas all real and artificial cowbird eggs were accepted. The lack of observed parasitism, therefore, was not an artifact of cowbird egg rejection. We suggest the lack of parasitism on Eastern Wood-Pewees at Delta Marsh reflects a partial asynchrony between the parasite's and host's laying seasons and possibly the high placement of pewee nests. However, these factors may not explain the lack of parasitism at all other sites in North America or on other Contopus species.
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Vol. 75 • No. 2