The tail morphology of many birds is shaped by both natural and sexual selection. Models of tail aerodynamics predict that functions related to flight naturally select for moderately forked tail shapes, constraining the tendency of sexual selection to generate interspecific morphological diversity. Moreover, models predict that small birds will have low variation in tail morphology. Although hummingbirds (family Trochilidae) depend exclusively on flight for locomotion, the diversity of their tail morphology is among the greatest in all bird families. Hummingbird tail length exhibits positive allometry, scaling as approximately mass^0.5, which causes tail surface area to scale as mass^1.0. Forked tail morphology arises at least 26 times in the clade, and forked taxa tend to be sexually dimorphic, whereas species with graduated tails tend to be monomorphic. Small hummingbird species exhibit higher variation in tail morphology than large species. These results suggest that moderately forked tail morphology tends to arise via sexual selection in hummingbirds and that large species are more constrained in tail morphology than small species. Both patterns are inconsistent with current models of how bird tails function. This suggests that, in terms of aerodynamics, hummingbird tails function differently from the tails of other birds.