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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 swainsonii that breed north of Mexico begin prebasic body molt on the breeding grounds and appear 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 Mexico 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 …