Contrasts in Scheduling of Molt and Migration in Eastern and Western Warbling-Vireos

Abstract

Using museum specimens, we determined that Eastern (Vireo gilvus) and Western (V. swainsonii) warbling-vireos have contrasting schedules for their prebasic molt relative to fall migration. Adult gilvus replace their primaries rapidly (38 days) and complete prebasic molt after breeding but while still on the breeding grounds. In contrast, adult swain- sonii that breed north of Mexico begin prebasic body molt on the breeding grounds and ap- pear to continue it during migration. These birds leave the United States and Canada before initiating remigial molt, and our data suggest that many of them stop in northwestern Mex- ico to begin replacing flight feathers. Adult swainsonii take longer to replace their primaries (55 days) than do adult gilvus, presumably because most or all of their migration has been accomplished prior to the initiation of molt. Hatching-year swainsonii that breed north of Mexico remain in the United States and Canada until their first prebasic molt (body only) is completed, and thus are present long after adults have left. Both species undergo a preal- ternate molt of body plumage, and occasionally innermost secondaries and the innermost pair of rectrices, on the wintering grounds. Our results, along with previous studies of molt and migration, highlight the importance of the southwestern United States and northwestern Mexico to the nonbreeding biology of several species of passerines that breed in western North America. Received 21 January 1997, accepted I July 1997.