Phenology and brown-headed cowbird parasitism

The brown-headed cowbird (Molothrus ater) is an obligate, generalist brood parasite that reduces the reproductive success of its many host species, but the magnitude of that reduction remains unknown. To address this question, I used a combination of field and laboratory experimentation, along with quantitative modeling, to evaluate the impact of parasitism on two local hosts in the Colorado Front Range: the primary host, the Plumbeous Vireo (Vireo plumbeus) and the least-parasitized host, the Western Wood-pewee (Contopus sordidulus). Results suggest that phenology is the main driver of cowbird host preference and parasitism impact. Temporal asynchrony in host breeding initially partitions hosts, which is then reinforced by host behavior. Paradoxically, Vireos, the negatively impacted primary cowbird host, are able to circumvent some of those impacts through phenology; predated vireos are forced to renest later in the season when parasitism has subsided, ultimately resulting in a higher success rate