Joint effects of social interactions and environmental challenges on physiology, internal microbiome, and reproductive performance in tree swallows (Tachycineta bicolor)

The social environment that individuals experience appears to be a particularly salient mediator of stress resilience, as the nature and valence of social interactions are often related to subsequent health, physiology, microbiota, and overall stress resilience. Relatively few studies have simultaneously manipulated the social environment and ecological challenges under natural conditions. Here, we report the results of experiments in wild tree swallows (Tachycineta bicolor) in which we manipulated both ecological challenges (predator encounters and flight efficiency reduction) and social interactions (by experimental dulling of a social signal). In two experiments conducted in separate years, we reversed the order of these treatments so that females experienced either an altered social environment followed by a challenge or vice-versa. Before, during, and after treatments were applied, we tracked breeding success, morphology and physiology (mass, corticosterone, and glucose), social interactions via an RFID sensor network, cloacal microbiome diversity, and fledging success. Overall, we found that predator exposure during the nestling period reduced the likelihood of fledging and that signal manipulation sometimes altered social interactions, but little evidence that the two categories of treatment interacted with each other. We discuss the implications of our results for understanding what types of challenges and what conditions are most likely to result in interactions between the social environment and ecological challenges.