• conservation;
  • experimental parasitism;
  • host–parasite interactions;
  • sex ratio

Inequality in male and female numbers may affect population dynamics and extinction probabilities and so has significant conservation implications. We previously demonstrated that Brown-headed Cowbird Molothrus ater brood parasitism of Song Sparrows Melospiza melodia results in a 50% reduction in the proportion of female host offspring by day 6 post-hatch and at fledging, which modelling demonstrated is as significant as nest predation in affecting demography. Many avian brood parasites possess special adaptations to parasitize specific hosts so this sex-ratio effect could be specific to the interaction between these two species. Alternatively, perturbations associated with brood parasitism per se (e.g. the addition of an extra, larger, unrelated nestling), rather than a Cowbird nestling specifically, may be responsible. We experimentally eliminated the effects of Cowbird-specific traits by parasitizing nests with a conspecific nestling rather than a Cowbird, while otherwise emulating the circumstances of Cowbird parasitism by adding an extra, larger (2-day-older), unrelated Song Sparrow nestling to Song Sparrow nests. Our parasitism treatment led to few host offspring deaths and no evidence of male-biased sex ratios by day 6 post-hatch. However, after day 6, female nestling mortality rates increased significantly in experimentally parasitized nests, resulting in a 60% reduction in the proportion of females fledging. Cowbird-specific traits are thus not necessary to cause female-biased host nestling mortality and far more general features associated with Cowbird parasitism instead appear responsible. Our results suggest, however, that Cowbird-specific traits may help accelerate the pace of female host deaths. The conservation implications of our results could be wide reaching. Cowbirds are unrelated to all their hosts, are larger than the great majority, and a Cowbird nestling's presence can mean there is an extra mouth to feed. Thus, sex-biased mortality in parasitized nests could be occurring across a range of host species.