COSTS OF PARASITISM INCURRED BY TWO SONGBIRD SPECIES AND THEIR QUALITY AS COWBIRD HOSTS

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Abstract
We measured the costs of Brown-headed Cowbird (Molothrus ater) parasitism incurred by Field Sparrows (Spizella pusilla) and Indigo Buntings (Passerina cyanea). We predicted that the frequent occurrence of nest desertion as a response to cowbird parasitism in Field Sparrows would be reflected by a higher cost of parasitism for that species. We also compared growth and survival of cowbird nestlings between hosts, predicting that they would do poorly at Field Sparrow nests because the latter appear to be avoided by cowbirds. Both species experienced reduced body mass gain in parasitized broods, but only Indigo Bunting suffered reduced tarsus growth. Both species experienced reductions in clutch size, hatching success, and nestling survival due to parasitism, but these losses did not differ among the two hosts. Multiple parasitism did not affect hatching success or nestling survival more than single parasitism for Indigo Buntings. Once accepted, cowbird offspring fared equally well in nests of both s...