Molt and Migration in the Northern Rough-Winged Swallow

Abstract

We describe the prebasic molt of Northern Rough-winged Swallows (Stelgidop- teryx serripennis) using museum specimens. Birds from eastern North America initiate their flight-feather molt about 40 days earlier than birds from west of the Rocky Mountains, sug- gesting that ecological differences between the two populations affect their molt schedules. In both populations, juveniles start their flight-feather molt I to 4 weeks later than adults, but the timing of body molt does not differ for adults and juveniles. Molt occurs simultaneously with the fall migration in both populations. However, eastern birds interrupt their migration when they reach the northern coast of the Gulf of Mexico. Here, they spend about two months finishing their flight-feather molt before crossing the Gulf in October and November. Molt and migration often are concurrent in diurnally migrating swallows, but the two activities do not occur simultaneously when migration must be sustained without interruption. Received 15 May