Explaining intra- and interspecific variation in sociality requires understanding the underlying ecological factors that promote or preclude grouping and the fitness consequences of group living. Colonial nesting, the dense concentration of individuals during breeding, offers an excellent opportunity for testing hypotheses to explain sociality and its variation. I studied the Hispaniolan Woodpecker (Melanerpes striatus), a facultative colonial breeder, in the Dominican Republic with the following goals, corresponding to the three chapters of this dissertation: (1) identify the ecological resources that explain colony size variation; (2) test for the effects of colony size on reproductive success with the goal of distinguishing the hypotheses of grouping for socially derived benefits from aggregations that form due to the spatiotemporal clustering of resources; and (3) determine whether coloniality impacts the genetic mating system by increasing the incidence of conspecific brood parasitism and extra-pair paternity. In Chapter 1, I show that Hispaniolan Woodpecker colony size is explained in part by the number of cavities on a tree and a nesting tree’s status: dead Roystonea palms hosted larger colonies than live palms. Dead palms were relatively rare on the landscape but were potentially higher quality because they were easier to excavate and immune to nest loss via cavity flooding. In Chapter 2, I show that, per nesting attempt, clutch size, hatching success, partial brood loss, and the number of fledglings produced were unrelated to colony size. However, nesting success, fledging one or more young vs. complete failure, was positively associated with colony size. Additionally, annual nesting success and annual fledging success were not associated with colony size. In Chapter 3, I show that Hispaniolan Woodpeckers were socially and genetically monogamous. Conspecific brood parasitism, though not detected genetically, was suspected in three nests based on aberrant changes in clutch size. Together, these chapters suggest that resource availability (dead trees and old cavities) likely promotes coloniality while there are minimal costs and very modest (if any) benefits of sociality for the Hispaniolan Woodpecker.