Meeting Abstract
Behavioral ecologists oftentimes use proxies for fitness because fitness can be difficult to measure directly in the wild. For example, nestling mass prior to fledging predicts survival into subsequent years in many songbirds, suggesting that mass may be a good proxy of quality or fitness. However, the mechanisms underlying this phenomenon are unclear. We hypothesized that immune function may influence offspring quality and survival, and specifically, we predicted that smaller offspring may be less able to mount an effective immune response, thereby negatively influencing survivorship. We tested our hypothesis in a wild population of tree swallows (Tachycineta bicolor), in which we used hematological gene expression of the inflammatory cytokine interleukin-6 (IL-6) as a measure of each chick’s ability to mount an acute phase response. We found that chicks with ectoparasites were smaller in mass on day 12 post-hatch, they had higher levels of IL-6 gene expression, and they were less likely to survive to fledging, compared to unparasitized chicks. We also report an interaction between parasite status, mass, and IL-6 mRNA abundance, such that mass and IL-6 are positively correlated among parasitized chicks, but there is no such relationship among unparasitized chicks. These findings suggest that, when the immune system is activated (i.e., in the presence of ectoparasites), larger chicks may indeed be better at fighting off diseases, thus accounting for their greater survivorship. Our results shed light on the proximate mechanisms linking nesting growth, immunity, and survivorship, and they highlight how parasites may influence the degree to which offspring mass is a good proxy for quality in songbirds.
