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Communal roosting is a taxonomically widespread phenomenon, with considerable variation in patterns of roost dynamics. Some organisms roost together in different locations each night (or day), whereas others roost in traditional locations each night, sometimes switching between roosts in the roost network. The behaviors that drive roost-site selection and the resulting patterns are not well understood. We created an individual-based model that simulates the daily aggregation of organisms into communal roosts. In the model, individuals move according to a movement rule integrating 2 independently adjustable behavioral drivers: roost fidelity, which leads individuals back toward their previous nights’ roost, and conspecific attraction, which leads individuals to congregate toward nearest conspecific neighbors. The model predicts that variable levels of aggregation will emerge under different combinations of these drivers, ranging from no aggregation to complete aggregation of individuals into their previous roosts. We tested to see which combination of drivers best predicts patterns of roost use in our study system of Tree Swallows (Tachycineta bicolor) in southeastern Louisiana in fall. Using Doppler weather radar data, we show that Tree Swallow roost sites remain consistent from night to night, and the birds return to one of several traditional roosts. Using radio telemetry, we show that individuals switch between these traditional roosts at minimum 22% of the time. Our results suggest that the formation of large communal roosts in Tree Swallows is driven by a combination of moderate conspecific attraction and strong, but not perfect, roost fidelity.