Life-history strategies are adaptations in behavior, physiology, and anatomy that influence survival and reproductive success. Variation in life-history strategies is often determined by adaptations to environmental conditions and trade-offs with sexually-selected signals. One of the aspects controlling life-history trade-offs is the endocrine system. Testosterone is a hormone that mediates several key aspects of male reproduction, yet little is known about the causes and consequences of variation in testosterone. Using rufous-collared sparrows (Zonotrichia capensis), a Neotropical songbird with a wide distribution, I explored geographical patterns of variation in testosterone levels and infection by haemosporidians, a type of blood parasite. I found that testosterone did not vary with elevation, nor predict haemosporidian infection, but males in breeding condition were more likely to be infected (Chapter I). High levels of testosterone have been associated with an increased number of sexual contacts and can suppress the immune response, thus it may increase the risk of sexually transmitted infections. By studying the communities of bacteria that reside in the cloaca of birds, I found that they were different depending on testosterone levels, and that high-testosterone males had higher relative abundance of Chlamydiae, a class of intracellular pathogens (Chapter II). During the breeding season there is an increase in physical contacts among individuals, testosterone levels increase in males, and there are additional energetic demands, all of which can increase exposure to bacteria or facilitate infection. I compared the cloacal microbiome of the same individuals between breeding and non-breeding seasons, and found that in males, but not in females, bacterial richness and phylogenetic diversity increased when birds were in reproductive condition. This suggested that the cloacal microbiome in birds is dynamic and responsive to breeding condition and sex of the host (Chapter III). Lastly, I synthesized the most relevant findings and suggested directions for future work (Chapter IV). I conclude that variation in testosterone is not always associated with immune suppression, and that the links among reproductive physiology, behavior, and the microbiome can provide insight into the evolution of life-history strategies.