Temporal variation in avian brood parasite condition and reproduction seem to affect host–parasite dynamics. Few studies, however, consider dynamics from the perspective of the parasite. Here we examined how brood parasite body condition and reproductive output vary both seasonally and annually and investigate the resultant impacts on nest parasitism rates of a host species. In the breeding seasons of 2011 and 2012, we collected female Brown-headed Cowbirds (Molothrus ater) from Fort Hood, Texas, and conducted morphometric, phenotypic, and physiological measurements on carcasses. During the same period, we also monitored nests of Black-capped Vireos (Vireo atricapilla) and recorded parasitism occurrence. Based on an analysis of >400 cowbirds, we found that cowbird body condition was significantly lower in 2012 than in 2011. Fewer females developed follicles in 2012 and follicle development was substantially delayed. Further, nest parasitism rates were significantly lower in 2012 and vireo nest success was significantly higher. The substantial variation we observed in cowbird body condition, follicle development, and parasitism may be related to a record-breaking drought that occurred in 2011. Cowbirds appeared to suffer negative carryover effects from the drought, likely due to reduced food resources, although similar effects on vireos were not observed. Detrimental effects of drought on cowbirds but not on vireos may have significant implications for host–parasite dynamics under changing climate conditions.
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Vol. 135 • No. 4