Competition Begins at an Early Age

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Science  13 May 2005:
Vol. 308, Issue 5724, pp. 927
DOI: 10.1126/science.308.5724.927a

The nestlings of brood parasites such as cuckoos and cowbirds, even if unsuccessful at ousting outright the eggs or nestlings of the host bird, compete with the host nestlings for provisioning by the parents. As a consequence of this competition, host nestlings frequently do not survive to fledge.

In a study of brown-headed cowbirds that parasitized song sparrow nests in Canada, Zanette et al. find that such mortality can result in sex-biased survival. The sex ratios of song sparrow nestlings and fledglings in parasitized nests differed significantly from those in unparasitized nests, with a much lower proportion of female chicks surviving in the former. In mixed-sex broods in unparasitized nests, female song sparrow chicks are already at a developmental disadvantage compared to females in single-sex broods, and the presence of cowbird chicks appears to exacerbate this intraspecific, intersexual competition. Thus, in areas where cowbirds are common, the brood parasites have the potential to affect song sparrow demography and sex ratio. These findings confirm recent theories that suggest that parasites and predators can alter the sex ratio of their host and prey populations. — AMS

Ecology 86, 815 (2005).

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