Examination of 1622 specimens indicates that North American Falconiformes show a wide variety of remigial (primary and secondary) replacement strategies, detectable throughout the year by evaluation of replacement patterns in the wings. Most Falconidae undergo complete prebasic molts whereas most Accipitridae display retained secondaries or show stepwise molt replacement patterns (“Staffelmäuser”). Among individuals exhibiting Staffelmäuser, minimum age can be inferred up to 5 years (fifth-basic plumage) by the number of “replacement waves” present among the primaries. It may also be able to infer breeding status during the previous summer by “suspension limits,” resulting from the interruption of molt during breeding. Among Accipitridae, Staffelmäuser occurred in species with greater mass, higher wing loading, longer migration distance, and more open rather than wooded foraging habitats: species that experience time constraints on molting and incur greater costs from large gaps in the wing. Thus, this study supports both the “time-constraints hypothesis,” suggesting that Staffelmäuser is a consequence of insufficient time for a complete annual molt, and the “aerodynamic hypothesis,” suggesting that Staffelmäuser reflects an adaptive need to replace as many feathers as possible without inhibiting flight efficiency. Time constraints may have been a proximate cause of Staffelmäuser among Falconiformes, with improvements to flying efficiency being an ultimate adaptive benefit.