Nest failure paradoxically leads to higher fledgling success

Background/Question/Methods The Brown-headed Cowbird (Molothrus ater) is an obligate brood parasite that reduces the reproductive success of many of its host species. Cowbird-host research over the past few years has focused heavily on determining the relative effects of parasitism and predation. However, despite extensive efforts, it is still poorly understood to what extent cowbirds reduce host reproductive success. To better address this question, we have created a mechanistic model, based on a 3-year data set, to calculate and assess the relative costs of parasitism and predation for one host species, the Plumbeous Vireo (Vireo plumbeus). This vireo is the primary host of cowbirds within our study area, the Front Range of Colorado, USA. Results/Conclusions The results from this work show that the costs of both parasitism and predation depend sensitively on the Julian date on which eggs are layed. Specifically, cowbirds (at least within the Colorado Front Range population) have synchronized their reproductive cycle around the same day that vireos begin to lay eggs. Under these circumstances, the model demonstrates that cowbird parasitism decreases vireo nest success by 14.9%. However, the model also demonstrates a more interesting result regarding the influence of predation on these populations. When a vireo's nest fails due to predation or environmental destruction (i.e. severe weather), renesting occurs about two weeks after the temporal peak in cowbird parasitism. After failure of a first nest, the model suggests that vireos experience an increase in total nest success (total number of fledglings leaving the nest), from 25% to 40%, because they are released from cowbird parasitism. These results highlight the importance of temporal scale when assessing the consequences of parasitism and predation on avian reproductive success.