Sexual size dimorphism of brood-parasitic nestlings does not affect host chick survival, size or fledging phenology

Intraspecific variability in parasite virulence holds significant ecological and evolutionary implications as it can result in uneven costs incurred by individuals of a host species. Obligate brood parasites, birds that lay their eggs in the nest of another species and do not raise their own young, differ in interspecific virulence, as some species kill all host nestmates directly while other species do not. However, variation in the intraspecific virulence of a brood-parasitic species has rarely been investigated. One source of this variability could arise through sexual size dimorphism, because many brood-parasitic species are sexually dimorphic in size starting at the nestling stage, and nestling size often impacts competitive ability in the brood. Here, we investigated the sex-specific effects of nestling brood-parasitic brown-headed cowbirds, Molothrus ater, on one of their hosts, the prothonotary warbler, Protonotaria citrea. We predicted that larger male cowbird nestlings would cause host chicks to have lower survival and size and altered life history in the form of delayed fledging. Using experimentally parasitized broods, we found that although male brood-parasitic cowbird nestlings are heavier than females, there was no effect of cowbird sex on the survival, size or fledging phenology of the host nestlings. Furthermore, there were no differences in fledging phenology between the male and female cowbird chicks. Instead, we found that wing length, which was similar between the parasitic nestlings' sexes, was an important predictor for age and order of fledging. These findings show that the extent of development is critical for the timing of fledging of both parasitic and host species. Notably, the lack of effect of cowbird nestling sex on host chick survival and fledging suggests that the presence of a parasite is more influential for host nestlings than the size of parasites.