Infectious disease as a cause and consequence of phenotypic responses to challenge in a songbird species

Throughout their lives, animals are faced with numerous ecological challenges stemming from abiotic and biotic conditions of their environment. Phenotypic shifts in response to one challenge can have cascading effects on other organismal systems, with downstream implications for individual fitness. Infectious disease presents a significant ecological challenge for most organisms on earth. Additionally, how an animal responds to disease can be shifted by exposure to other ecological challenges. Thus, infectious disease can both present an ecological challenge itself or shift as a consequence of another challenge. In this work, I used experimental captive studies on wild-caught house finches (Haemorhous mexicanus) to elucidate how an animal might shift its phenotypes when presented with an ecological challenge. In the first experiment, I examined how nutritional stress during nestling development impacted the magnitude of house finch responses to the bacterial pathogen Mycoplasma gallisepticum (MG). Although nutritional stress limited mass gain in nestlings, individual responses to MG did not vary with nutritional stress, possibly indicating that the development of immune responses is resilient even in the face of suboptimal nutritional conditions. Next, I investigated infectious disease as a challenge in itself and asked how individual social preferences were shifted by MG infection. I demonstrated that MG-infected house finches showed augmented sociality relative to control birds, choosing to spend more time with a group of conspecifics than alone. Because this increased social preference was no longer present once birds recovered, this phenotypic change in sociality may have specific benefits for actively infected birds. Finally, my last experiment expands upon these results, exploring whether group-living particularly benefits infected birds by offsetting two common fitness costs of infection: reduced foraging abilities and decreased anti-predator responses. Here we found that group-living provides all individuals with improved foraging and anti-predator behaviors, with the strongest benefits of group-living apparent for infected finches. This suggests that augmented sociality in infected house finches has important implications for surviving infection, and potentially, for the spread of MG within populations. As animals continue to face increasing and novel ecological challenges, it is vitally important to understand individual responses to environmental challenges, which can have long-term effects for all levels of biological organization. In particular, my work highlights the role of social behavior as a potentially adaptive phenotypic response to infectious disease in wild animals. Taken together, my results demonstrate the importance of continuing to study infectious disease from multiple perspectives to better understand how animals will respond to a shifting world.