Costs of parasitism incurred by two songbird species and their quality as Cowbird hosts

We measured the costs of Brown-headed Cowbird (Molothrus ater) parasitism i ncurred 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 hi gher cost of parasitism for that species. We also compared growth and survi val of cowbird nestlings between hosts, predicting that they would do poorl y at Field Sparrow nests because the latter appear to be avoided by cowbird s. Both species experienced reduced body mass gain in parasitized broods, b ut only Indigo Bunting suffered reduced tarsus growth. Both species experie nced reductions in clutch size, hatching success, and nestling survival due to parasitism. but these losses did not differ among the two hosts. Multip le parasitism did not affect hatching success or nestling survival more tha n single parasitism for Indigo Buntings. Once accepted, cowbird offspring f ared equally well in nests of both species, but almost half of all cowbird eggs Laid in Field Sparrow nests were lost through nest abandonment. As par asitism costs to both species appear to be substantial, the rarity of nest desertion in Indigo Buntings may be due to other factors. Infrequent parasi tism of Field Sparrows is consistent with host avoidance by cowbirds but ot her explanations should be explored.