We used museum specimens to describe the timing and location of the postbreeding molt in Western Kingbird (Tyrannus verticalis), an insectivore that breeds in arid lowlands of western North America, where late summer conditions are exceedingly dry. Like many other western migrants, adult Western Kingbirds depart their breeding grounds and move to molt in the Mexican monsoon region. By contrast, juveniles stay on the breeding grounds in late summer, delaying their eccentric primary molt and body molt until after undertaking part of their fall migration. The high number of juvenile specimens collected on the breeding grounds in late summer confirms that the decrease in adults, measured as the percentage of all Western Kingbirds that are adults, is not an artifact of inactive collectors. We also demonstrate adult departure using museum databases to calculate the percentage of all passerines collected in breeding areas outside the southwestern molting grounds that were adult Western Kingbirds. The close correlation between these indices validates the use of total number of passerines as an index of the collecting effort targeting a specific passerine. Our results provide another example of the importance of the Mexican monsoon region for molting passerines, highlighting the need to preserve habitat in this region.