AimWe investigated the contribution of parasitism by the Brown-headed Cowbird (Molothrus ater) to its host population dynamics at a subcontinental scale. We predicted that the growth rate of cowbirds would be most strongly related to the abundance of forest insectivorous hosts and that landscape fragmentation would have opposite effects on the parasite and its hosts. Furthermore, due to the species' sedentarity and low rates of adult dispersal, we expected that cowbird population dynamics would be structured at local, rather than large spatial scales.LocationThe Great Plains, United States.MethodsWe modelled jointly the population dynamics of cowbird and its 58 most common hosts, accounting for effects of landscape fragmentation and cowbird dispersal. We used Bayesian inference to evaluate this model on 76 routes of the North American Breeding Bird Survey (BBS), surveyed each year from 2001 to 2011. We then analysed covariations between the demographic dynamics of cowbirds and their hosts in a multivariate ecological trait space.ResultsContrary to most of its hosts, the cowbird had positive growth rates in man-disturbed, fragmented landscapes, which covaried at small spatial scales. The large-scale effects of parasitism were limited, as few host species affected, or were affected by cowbird growth rates. However, significant cowbird–host relationships were structured by ecological traits: cowbird growth rates were most increased by the abundance of ecological generalists, while hosts were affected by cowbird abundance irrespective of their position in the functional space.Main conclusionsCowbirds parasitize opportunistically the hosts occurring in disturbed landscapes. The large-scale effects of nest parasitism therefore depend probably on landscape structure and other environmental factors operating at local scales. We suggest that efforts to counter the local and continental declines of many passerine species should include parasitism among the biotic components of landscape disturbance.Read less <