Immature individuals are generally more susceptible to infection and disease than adults. Study of humans and domesticated animals suggest that the immune system mediates this age-related pattern. Despite the rapid growth of ecological immunology in the last few decades, we still know very little about age-related patterns of immunity of organisms in their natural environments. In this study we compared several aspects of immune structure and function between free-living nestling and adult tree swallows (Tachycineta bicolor). We measured several components of both the innate and acquired arms of the immune system and predicted that nestlings would have reduced levels when compared with adults.