Abstract Dissertation Research: Host/Parasite Dynamics between Red- winged Blackbirds and Brown-headed Cowbirds PI: Jeffrey R. Baylis Co-PI: Ethan D. Clotfelter This study will examine the effects of brood parasitism by brown-headed cowbirds (Molothrus ater) on the reproductive success of red-winged blackbirds (Agelaius phoeniceus) and evaluate the importance of this selective pressure in modifying host behavior. Fieldwork will be conducted during March-July of 1996 and 1997 on a prairie-nesting population of red-winged blackbirds in Rock County, Wisconsin. This study has four aims. (1) Redwing nest defense effort will be measured using mounted female cowbirds presented at 3 distances from the nest. Vigilance by male redwings will also be related to the probability of parasitism. These data will be used to assess the extent to which redwing behavior has evolved as a response to cowbird parasitism. (2) The effect of cowbird egg-removal on redwing reproductive success will also be examined by combining clutch manipulations with experimental parasitism. (3) Redwing parental investment will be compared between parasitized (exposed to cowbird mount) and unparasitized (not exposed to cowb ird mount) nests. (4) Empirical estimates of redwing quality will be measured in the field and compared between parasitized and unparasitized nests, and data from 1983-1997 nest records will be used to determine if parasitized nests have intrinsically higher success rates than unparasitized nests. Nest success of naturally- and experimentally-parasitized nests will be compared to further test this host quality hypothesis. Results from this study will be widely applicable to other avian host/parasite systems, including sensitive species where experimental manipulation is impractical or impossible. By studying the dynamics of this relationship between closely-related species we can gain greater insight into the evolution of obliga te interspecific brood parasitism.