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The brown-headed cowbird (Molothrus ater) is a widespread, obligate brood parasite of North American passerine birds. In southern Manitoba, where hosts are sympatric with cowbirds, American robins (Turdus migratorius) ejected parasitic eggs from all experimentally parasitized clutches (N = 25) and no eggs were accepted for more than four days. In contrast, robins in northern Manitoba, an area where cowbirds do not breed, accepted parasitic eggs in 33% of nests (N = 18) for at least five days. Acceptance of experimental cowbird eggs by a second host, the yellow warbler (Dendroica petechia), was similar in allopatric (100% of 20 nests) and sympatric (88.6% of 35 nests) populations, but models of a female cowbird elicited greater nest defense by warblers in the area of sympatry. Neither host rejected eggs of conspecifics, thus, rejection of cowbird eggs was not an epiphenomenon of conspecific brood parasitism. These results support the hypothesis that recognition of cowbirds and their eggs evolved as adaptations to counter cowbird parasitism and not some other selection pressure. The expression of anti-parasite defenses by some individuals within allopatric populations further suggests these traits may be controlled genetically but persist in such areas either through the continued introgression of rejecter genes from sympatric populations or because of the low cost of rejection behavior when parasitism is absent or rare.

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