Corey T Callaghan , Stephanie D Jurburg 

Background: The avian microbiome plays an essential role in host development, health, and behavior, but while microbiomes of captive birds have been extensively studied, little is known about how life history traits influence the resident microbial diversity of individuals and of species in wild birds. Host traits may shape their associated microbiomes by modulating the exposure of the host to microbes (e.g., through dispersal), or by selecting or removing subsets of the community, and they can affect the diversity of individuals (alpha diversity) or of the entire population (gamma diversity). To explore the relationship between interspecific traits and microbiome diversity in birds, we synthesized 773 microbiome samples and host trait data across 133 bird species and explored whether traits related to exposure to conspecifics (flock size, global abundance), environmental microbiomes (trophic level, primary habitat, primary lifestyle, body mass), or describing the range of exposure to microbes, or dispersal, (habitat breadth, range size) influence interspecific differences in individual or population-level diversity in bird-associated microbiomes.

Results: We found that traits related to exposure to environmental microbiomes (habitat, primary lifestyle, trophic level), and global abundance were the strongest predictors of differences in the composition of the bird microbiomes across species. Furthermore, we found that traits related to microbiome dispersal (range size and habitat breadth) were positively related to gamma, but not alpha diversity, highlighting that dispersal-related traits may be acting on the population level. Traits related to exposure to conspecifics were negatively related to alpha and gamma diversity, suggesting that social exposure is not a mechanism for microbial dispersal into hosts. Finally, we found higher richness, but evidence of biotic homogenization in the microbiomes of birds inhabiting human modified systems.

Conclusions: Our study demonstrates the importance of studying interspecific differences in microbial diversity to understand the ecological drivers of host-associated microbiomes, and highlights the potential of syntheses approaches for doing so.