Discussion

This study serves as an initial evaluation of the usefulness of the cowbird as a model species in studying the relative importance of environmental exposure and host genotype on composition of the gut microbiome. In addition, our results offer the possibility that life-history related differences in microbiome development may explain previously poorly understood differences in wildlife population dynamics. We predicted that the microbiome of adult cowbirds would display significantly higher beta diversity than that of related non-parasitic species. Our findings showed significantly higher beta diversity among cowbird cloacal samples compared to those of redwing blackbirds, which is consistent with young cowbirds acquiring microbial elements from diverse host parent species and having these microbes maintain their relative abundance into adulthood. Our results also suggest that the life history strategy of the cowbird as a host-generalist brood parasite may serve as a useful natural model for how life-history dynamics influence microbome structure.

Avian brood parasites offer an opportunity to observe a natural experiment in development of the gut microbiome in the presence of foreign microbes as well as a useful model species in which to experimentally introduce foreign elements to study resilience and homeostasis of the gut microbiome.  The biomedical community is investigating the development, composition, and functions of the gut microbiota of the human species with keen interest (Consortium 2012).  Studies of perturbation and resilience of the gut microbiome, including the possible deleterious effect of overuse of antibiotics, are being pursued for insight into several chronic diseases, including Irritable Bowel syndrome (IBS), Crohn’s Disease, diabetes, and obesity (Willing, Antunes et al. 2011).  The gut microbiota of commercial species is beginning to be a concern with recognition that animal husbandry practices that routinely feed antibiotics likely have significant consequences for the health of these animals and the health has of the human population that consumes them (Blaser 2014).

Given the role of the gut microbiome in immune responses, future investigation is warranted into the possibility that the composition and/or development of the cowbird gut microbiome has a role in the cowbird’s display of unusually effective immune responses.  Cowbirds showed significantly greater resistance to infection with the West Nile virus than do three closely related, nonparasitic blackbird species (Reisen and Hahn 2007, Hahn and Reisen 2011).  Greater resistance to infection with a virulent pathogen like WNV, an invasive species that emerged in the US in 1999 and to which no domestic songbirds had previously been exposed, is a strong indicator of unusually effective immunity.   This phenomenon of greater immunity in this cowbird has been documented with additional experimental infections of the cowbird and related blackbirds to two virulent native viruses, WEEV and SLEV, and again the cowbird displayed more effective immune responses. 

Our hypothesis of the cowbird’s greater immunity suggested that the evolution of more effective immunity in cowbirds occurred as a byproduct of exposure to foreign microbes in the nests of the species they exploit. To characterize the cowbird’s immune responses, we also carried out comparative studies using two other t parasitic cowbird species, shiny, and bronzed, to see whether the phenomenon of greater immunity was characteristic of all of the parasitic cowbird species. We found more effective immune responses in all parasitic birds we tested, with degree of immunity corresponding to whether the cowbird was a moderate or extreme host generalist. Moderate host generalists, like the bronzed cowbird, were exposed historically to only 12-20 host species and thus lower selection from foreign microbes on the cowbird’s immune responses. A bit on the direction this should take