Host Diet Preference Drives Diversity and Composition of Gut Microbiota in Captive Birds

Gut microbiota (GM) plays a vital role in host physiology, yet our understanding of the factors driving GM variability in birds remains incomplete. Previous research has provided mixed support for different predictors of bird GM variation, possibly due to the high heterogeneity of avian GM combined with the strong influence of environmental factors on its composition. To suppress the role of these confounding factors, we focused on interspecific GM variation in birds from captive populations, with the aim of clarifying the role of diet and phylogeny. Using 16S rRNA amplicon sequencing, we analysed the GM of 36 bird species from 14 orders, focusing on variability in GM diversity and distribution of individual bacterial constituents. We found that host phylogeny only had limited influence on GM diversity and composition. On the other hand, we identified diet preference of host species as a significant predictor of GM diversity and composition, with herbivorous species exhibiting higher GM alpha diversity than carnivorous species. Furthermore, we observed a converging pattern of GM composition among phylogenetically unrelated carnivorous species, driven by increased abundance of microbial taxa that mostly had an undetermined role in host physiology. This contrasts with obligatory anaerobic bacteria from the phylum Bacteroidetes, and other commensal bacteria, observed with increased abundance in hosts preferring carbohydrate-rich vegetarian diets. Overall, our findings emphasise host diet preference as an important factor determining GM diversity in birds, explaining the convergence of GM composition in phylogenetically distant host species.