What are the costs of raising a brood parasite? Comparing host parental care at parasitized and non-parasitized broods

Young obligate brood parasitic birds impose a cost on their hosts by exploiting care from unrelated foster parents. While raising parasitic young is detrimental to hosts' fitness because it reduces the clutch size and the fledging success of the original brood, the potential relationship between parental care provided for parasitized broods and foster parents' future reproductive output remains poorly understood. Using video-recordings of eastern phoebe (Sayornis phoebe) nests, a proportion of which were naturally parasitized by brown-headed cowbirds (Molothrus ater), we quantified and compared different aspects of parental behaviors of this common host species. We found that nest attendance rates, but not brooding, were positively related with the total number of nestlings (brood size) and the proportion of cowbirds reared in a brood (parasite load). However, phoebes' parental behaviors were not related to cowbird parasitism per se probably because parasitized broods overall contained fewer nestlings than did non-parasitized broods. Based on our correlational data we estimate that phoebe parents deliver 3.6 times greater parental effort to raise a parasitic cowbird chick than one of their own offspring. Differences in parental care provided for varying brood sizes and parasite loads may provide a mechanism to explain how raising brood parasitic young is related to reduced residual reproductive effort in Eastern phoebes and perhaps in other host species.