Mounting immune responses against pathogens and parasites is a notoriously costly endeavor. And thus, immune responses are subject to physiological trade-offs that balance resource and energy investment between immunity and other life history traits.While the immune system is considered a critical component of organismal life history, we know little about how life history trade-offs shape the various immune phenotypes we see in nature. In this dissertation, I explore what shapes immune defenses from three perspectives: drivers of within-population variation in immune function, drivers of between-population variation in immune function, and how environmental conditions shape the development of immune defenses. Throughout this work, I measured bacteria killing ability (BKA) to measure the effectiveness of immune function in tree swallows (Tachycineta bicolor). In chapters 1 and 2, I show that life history traits like timing of breeding can determine whether immune function is subject to a trade-off with reproduction, and how responsive it is to chronic elevations of glucocorticoids, the main hormonal mediator of the stress response. In chapter 3, I investigate how bacteria killing ability varies geographically along a gradient of environmental unpredictability. In this study, I show that immune function declines with increasing environmental unpredictability, potentially due to increasing brood reproductive value in locations with more unpredictable environmental conditions. Locations with higher reproductive value are more likely to allocate more resources in reproduction than immunity because elevated environmental unpredictability would reduce the chances to breed again later. Finally, in chapter 4, I investigate how environmental conditions during development, specifically exposure to cold temperatures - one of the outcomes of increased environmental unpredictability - drives resource allocation strategies that prioritize thermoregulation at the cost of immune investment. I found that cold-exposure during incubation made nestlings better thermoregulators, which had not been observed in altricial birds like tree swallows, but that it did not come at a cost to immune function. Overall, I show that immune investment and function is broadly dependent on life history strategies that may vary among individuals and across populations. Thus, my work suggests that life history can shape disease vulnerability, reinforcing the role of immunity as a life history component.