Animals are ecological landscapes that host communities of microbes often referred to as microbiomes. These microbes can be transferred between individuals when they come into contact, such as during mating. Microbes that reside in or on any aspect of a host that becomes exposed to the reproductive tract or gametes comprise the reproductive microbiome. These microbes within the reproductive microbiome are important to overall host biology because they can influence host reproductive function, and thus play a role in shaping host ecology, evolution, and fitness. Though previous work has revealed much about the impact of beneficial and pathogenic microbes within the reproductive tract, much is left to be learned from describing the dynamic nature of the reproductive microbiome, and ultimately, how it affects host fitness. For my dissertation, I asked questions regarding how and why reproductive microbiome diversity varies among individuals. For instance, does reproductive microbiome diversity vary with respect to the number of mates or mating activity? Does reproductive microbiome diversity vary with host age or breeding stage? Are there fitness consequences associated with differences in reproductive microbiome diversity? To explore these questions, I studied tree swallows (Tachycineta bicolor), a socially monogamous bird in which both females and males engage in extra-pair mating activity. I focused on the cloacal microbiome as it is the site of contact during mating, and thus where microbes can be sexually transferred between individuals. I found that social partners did not have more similar cloacal microbiome diversity compared to other individuals in the same population, and that cloacal microbiome diversity was similar between sexes (Chapter II). By combining an observational approach with a hormone implant manipulation, I found that neither the number of sires per brood nor the increased mating activity of females significantly influenced cloacal microbiome richness or community structure. However, female age and breeding stage did significantly correlate with cloacal microbiome richness and community structure (Chapter III). Based on these findings, I hypothesized that the effect of mating activity on variation in the cloacal microbiome may only be detectable over a female's lifetime, and not within a single breeding season. In addition, I found evidence for a relationship between lay date and cloacal microbiome structure, after controlling for age. And I found that older females lay earlier in the season compared to younger, first-time breeding females (Chapter IV). These results provide support for a relationship between lay date and the cloacal microbiome and highlight the importance of age to this relationship. Lastly, I discussed future steps that can be taken to extend the framework established by my dissertation research, and thereby gain further insight into the factors shaping the reproductive microbiome (Chapter V). / Doctor of Philosophy / Animals host communities of microbes often referred to as microbiomes, and these microbes can be transferred between individuals when they come into contact, such as during mating. Microbes that reside in or on any part of a host that becomes exposed to the reproductive tract make up the reproductive microbiome. These microbes within the reproductive microbiome are important to an animal's biology because they can influence reproductive function, such as fertilization and pregnancy success. Though previous work has revealed much about the impact of beneficial and pathogenic microbes within the reproductive tract, much is left to be learned from describing the dynamic nature of the reproductive microbiome as a whole and how it affects an animal's reproductive success. For my dissertation, I sought to understand how and why reproductive microbiome diversity varies among individuals, especially in relation to mating. To explore these questions, I studied tree swallows (Tachycineta bicolor), a species of bird in which females and males will pair up to breed and rear young, yet both may mate with individuals other than their partner. I focused on the microbiome within the cloaca of birds as it is where contact is made during mating, and thus where microbes can be sexually transferred between individuals. I found that social partners did not have more similar cloacal microbiome diversity compared to other individuals in the same population, and that cloacal microbiome diversity was similar between females and males (Chapter II). Since tree swallows frequently mate with multiple partners and it is possible for bacteria to be acquired though each mating attempt, I proposed that it is important to consider the number of mates per individual when assessing the diversity of the cloacal microbiome. I then performed observational and experimental studies where I assessed the number of mates per female and manipulated female sexual activity with hormone implants. I found that neither the number of sires per brood nor the increased mating activity of females influenced cloacal microbiome diversity, however, female age and breeding stage were significantly associated with cloacal microbiome diversity (Chapter III). Based on these findings, I hypothesized that the effect of mating activity on variation in the cloacal microbiome may only be detectable over a female's lifetime, and not within a single breeding season. In addition, I found evidence for a relationship between lay date and cloacal microbiome structure, after controlling for age. And I found that older females lay earlier in the season compared to younger, first-time breeding females (Chapter IV). Taken together, these results provide support for a relationship between lay date and the cloacal microbiome and highlight the importance of age to this relationship. Lastly, I discussed future steps that can be taken to extend the framework established by my dissertation research and gain further insight into factors shaping the reproductive microbiome (Chapter V).