Gut microbiome–diet interactions in wild birds

Birds show global declines, and understanding the relationship between avian diet and fitness can both answer basic questions in physiological ecology and inform conservation efforts. Diet-induced changes to the gut microbiome, the collection of microorganisms and their functional genes and metabolites inside the gut, may be of particular importance to avian fitness as the gut microbiome provides a suite of beneficial roles for nutrition and immunity of the host. Furthermore, evidence is growing that the gut microbiome may impact animals' diet choices, which could have cascading impacts on avian fitness. Sequencing technologies allow both diet and gut microbial composition and diversity to be characterized from the same fecal sample, creating ripe opportunities to explore diet–microbiome relationships. In this mini-review we summarize the existing literature on the effect of diet category, diet shifts, and dietary diversity on the gut microbiome, and the potential for the gut microbiome to serve as a modulator of diet choice in wild birds. We list open questions in the field of avian diet–microbiome interactions and provide methodology considerations for designing studies to sample both diet and gut microbiomes. This mini-review provides a framework for understanding the reciprocal relationship between diet and gut microbiota in wild birds.