Abstract

As brood parasites, brown-headed cowbirds (genus Molothrus) have shown superior resistance to experimental infection with virulent pathogens including West Nile virus, St. Louis encephalitis virus, and western equine encephalitis virus. Given the potential role of the gut microbiome in the vertebrate immune system, brood parasites offer a unique opportunity to investigate the relative importance of nature versus nurture in microbial colonization of the gut and the potential for the gut microbiome to facilitate immune resistance. Young brood parasites hatch and develop in the nests of other species, which provide parental care. This allows cowbird parasite nestlings to avoid exposure to biological parents during early development when the gut microbiome is colonized or at least heavily influenced by exogenous microbes. Thus, we hypothesized that the population level gut microbiome of the brown-headed cowbird is a composite of species-specific microbes from a range of host species, which may contribute to their effective immune responses. To test this we compared the microbial communities of the cowbird and the red-winged blackbird, a close relative with similar diet, geographic range, and social behavior. We found beta diversity of cloacal microbiomes, was significantly higher in cowbirds than red-winged blackbirds, a pattern consistent with the hypothesis that young cowbirds acquire microbes from their foster parent species. However, cloacal microbiome membership did not reflect microbial membership the GI tract microbiome, suggesting that cloacal microbiomes are not good proxies for GI tract microbiomes. Regardless, the enhanced diversity of the cowbird cloacal microbiome suggests the cowbird’s exposure to other species during early development may affect the development of their microbiome and be an additional and previously unconsidered important selection pressure in the development of brood parasitism as a viable natural history.