Discussion
We investigated how dietary macronutrients affected host physiology and gut microbiota and characterized how feeding behavior and macronutrient selection were influenced by both immune activation and perceived infection risk. We found that shifts in dietary macronutrients can affect the abundance of some bacterial taxa and potentially alter relationships between gut microbial communities and immune and stress physiology. Additionally, a direct immune threat but not a perceived risk of infection altered the feeding behavior of birds through reducing the consumption of specific macronutrients, which could have implications for responding to and recovering from infection. Taken together these results suggest that both diet-induced shifts in the microbiome and infection-induced shifts in macronutrient selection occur in songbirds, and interactions between macronutrients, gut microbiota, and feeding behavior likely have important consequences for host health.