Mating and Parasitic Behavior of Brown-Headed Cowbirds

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ABSTRACT PI: ASHLEY PROPOSAL NUMBER: 9601201 Mating and Parasitic Behavior of Brown-headed Cowbirds The breeding biology of the brown-headed cowbird, Molothrus ater, will be investigated through a comprehensive field and laboratory study. Both the mating system and the parasitism practices of individual female cowbirds will be studied by assigning parentage to 450-500 cowbird eggs and nestlings collected at a field site at the Morton Arboretum in Lisle Illinois from mapped nest locations. This will be done by DNA microsatellite genotyping, which allows parentage to be determined by exclusion. The genetic data will be used to determine whether cowbirds at this study site mate monogamously, as has been previously reported, or promiscuously. Brood parasites such as the brown-headed cowbird are freed from parental care, which makes monogamy theoretically improbable. If it is determined that cowbirds are indeed monogamous, prevailing theories of avian mating systems may need to be reevaluated. The genetic analysis will also be used to test the hypothesis that female cowbirds restrict egg-laying to definable areas that are largely non-overlapping. Non-overlapping reproductive areas would suggest competition for a limited resource, host nests. This will be done by mapping the locations of offspring assigned to an individual female and comparing these to distributions of other femalesU offspring. Previous field work at this study site demonstrates that cowbirds differentially parasitize the nests of certain host species when multiple hosts are available. Several ecological factors including habitat, nest type, host size, and host defenses are strongly correlated with observed rates of parasitism. The proposed study will examine the extent to which these patterns reflect behavioral adaptations (e.g. host choice) of female cowbirds. Two related field studies will be conducted to determine if fem ales preferentially parasitize the highest quality host species and/or avoid low quality host species such as those known to eject cowbird eggs from their nests (RrejectorsS). The first field study will determine the relative abilities of different commonly parasitized host species to successfully foster cowbird nestlings. The relative qualities of different hosts as cowbird surrogates will be compared to observed patterns of parasitism to determine if high-quality host species are selectively parasitized. The second field study will determine parasitism rates of RrejectorS species. At our study site we have observed parasitism of three species of hosts known to eject cowbird eggs, indicating that female cowbirds are not completely avoiding these host species. By experimentally parasitizing the nests of these species, the rate of egg-ejection will be determined. From this measure and the observed rate of parasitism, we can ascertain whether cowbirds prefer acceptor host species over rejectors, or alternatively, fail to discriminate against these low-quality hosts. Demonstrating patterns of host choice is important both for understanding host-parasite coevolution and for developing conservation strategies to protect host species threatened by cowbirds.