Falco sparverius (American Kestrel) is a declining species that readily uses human-provided nest boxes. We test several hypotheses from the literature regarding nest microhabitat preferences of American Kestrels, including putative preferences for brighter cavities and cavities with a given compass orientation, both of which are believed to provide some thermal benefit. We placed light and temperature data loggers in 26 nest boxes in Massachusetts after young had fledged from occupied boxes. Cavity interior light and temperature were not correlated, and while nest-box orientation did affect cavity interior light levels, none of these variables was significantly associated with occupancy. Microhabitat variables may be more important in some geographic areas than in others; the results of our study suggest that existing hypotheses need experimental verification. It is possible that multi-region studies integrating the effects of ambient temperature and wind on cavity selection will better explain apparent differences in microhabitat preferences across the species' range.