Brood parasitism by cowbirds: risks and effects on reproductive success and survival in indigo buntings

We observed brood parasitism by brown-beaded cowbirds (Molothrus atsr) on indigo buntings (Passerma cyanta) and estimated die impact of parasitism on the success of the individual buntings in their current nests and in their future survival and reproduction. Rates of parasitism over 8 years were 26.6% in 1040 nests and 19.8% in 693 nests in two areas in southern Michigan. Risk of parasitism was high early in the season; half the bunting nests were begun after the end of the cowbird season. Risk was independent of female age, plant containing the nest, or habitat The immediate cost of parasitism was 1.19 and 1.26 fewer buntings fledged per nest. Bunting success was lower in parasitized nests with cowbird eggs (nests were more likely to be deserted or predated), lower when the cowbird nestling failed (nests were more likely to be predated), and lower when the cowbird fledged (fewer buntings fledged) compared to nonparasitized nests. Costs were due to removal of a bunting egg when die cowbird laid its own egg and to competition for parental care of the cowbird and bunting nestlings. Buntings that fledged from nests where a cowbird also fledged were only 18% as likely to survive and return to their natal area in the next year as buntings from nests where a cowbird did not fledge. Long-term effects of cowbird parasitism on adult breeding later in the season, survival to the next season, and reproductive success in the next season were negligible when compared between birds that reared a cowbird and birds that reared only a bunting brood, or between birds that were parasitized and birds that escaped parasitism. The results indicate little long-term cost of brood parasitism on individual fitness of adult buntings beyond the impact on the current nest and the survival of buntings that fledge from it; nearly all cost is to the parasitized brood. Kty words: breeding seasons, brood parasitism, brown-headed cowbirds, cost of parasitism, cost of reproduction, indigo buntings, individual fitness, Molothrus ater, natal philopatry, nest predation, nestling competition, Passerrna cyanta, postfledging survival, reproductive success. [Behav Ecol 9:64-73 (1998)]