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Bergmann's rule states that homeotherms are larger in colder climates (which occur at higher latitudes and elevations) due to thermoregulatory mechanisms. Despite being perhaps the most extensively studied biogeographical rule across all organisms, consistent mechanisms explaining which species or taxa adhere to Bergmann's rule have been elusive. Furthermore, evidence for Bergmann's rule in migratory animals has been mixed, and it was difficult to assess how environmental conditions across the full annual cycle impact body size until the recent miniaturization of tracking technology. Nightjars (Family Caprimulgidae), nocturnal birds with physiological and behavioral adaptations (e.g., torpor) to cope with the environmental extremes they often experience, offer a unique opportunity to elucidate the mechanisms underpinning Bergmann's rule. Many nightjar species are strongly migratory and have large breeding ranges, offering the opportunity to look at variation in potential drivers within and across seasons of the annual cycle. Furthermore, variation in migration strategy within the family provides an opportunity to separate adaptations for migration strategy from adaptations for thermal tolerance. In this study, we use cross-continental data from three species of nightjars (Common nighthawk, Eastern whip-poor-will, and European nightjar) to assess 1) whether migratory species in this clade adheres to Bergmann's rule, 2) which environmental factors are the best predictors of body size, and 3) the extent to which environmental conditions across the full annual cycle determine body size. For each species, we use breeding and winter location …