| The Brown-headed Cowbird (Molothrus ater) is an obligate avian brood parasite that can dramatically reduce reproductive success of its hosts, and has parasitized more than 220 species across its North American range. The cowbird's continuing range expansion and population growth has caused concern among conservation biologists. Although cowbirds have been the subject of hundreds of studies and two scientific symposia, several aspects of their reproductive biology have not before been documented. This dissertation addresses several such topics.; In Chapter I, I tested hypotheses of host selection, and used incomplete-block design ANOVA to examine the effects of host taxonomy and natural history traits on parasitism frequency among hosts at 60 sites across North America. Neither host quality nor host abundance were correlated with parasitism at any site tested. In forest habitat parasitism was influenced by host taxonomy, host body size, nestling period length, and foraging behavior. Vireonidae, Thraupidae, Parulidae, and Turdidae were most parasitized, followed by Cardinalidae and Emberizidae, then Fringillidae and Tyrannidae, and Icteridae were least parasitized.; Chapter II compares patterns of total cowbird egg investment, host nest availability and cowbird recruitment among hosts in a pinyon juniper avian community in New Mexico. Cowbirds distributed eggs more evenly among host species (Simpson's index: 1-D = 0.857) than expected based solely on host nest availability (1-D = 0.796). Eleven species were parasitized with 69.5% of total cowbird egg investment divided equally between four species. Cowbird productivity was diverse with respect to host species (minimum estimate: 1-D = 0.65), although Western Tanagers contributed an estimated 31–41.1% of overall cowbird productivity.; Chapters III and IV describe a trade-off between cowbird commuting distance and egg production. At a site in New Mexico, I used radio telemetry to show that cowbirds commuted daily upto 13km each way between breeding and feeding areas. Egg production, determined by counts of postovulatory follicles, of long-distance commuters was half that of short-distance commuters. These results have direct conservation implications for management strategies aimed at reducing parasitism in cowbird hosts by eliminating cowbird feeding sites. |
