Annual replacement of flight and body feathers is a major time and energy demand in the life of small birds. If birds have a hard time finding the resources they need to molt, they may adopt a strategy of migrating to an area where resources are less scarce before molting. Since 1990, researchers at the Burke Museum have led the discovery of "molt migrations" in western songbirds that winter south of the US border. These migrant birds breed in the western United States, but then move immediately after breeding to molting grounds in the southwestern US and western Mexico. Molt migrations characterize many songbirds that breed in the lowlands of the interior west, including Bullock's Orioles, Western Tanagers, Black-headed Grosbeaks, Lazuli Buntings, Ash-throated Flycatchers, Lucy's Warblers, Lark Sparrows, and Western Kingbirds. In contrast, migrants that breed in the mountains, such as Hermit and Townsend's Warblers and Cassin's Vireos, tend to move up slope where moist conditions allow them to molt in their breeding range before moving south.
In the American West most annual precipitation comes as winter rain or snow. Thus a flush of spring productivity supports breeding, but as plant growth stops in mid and late summer, food production for many birds also declines. Departing the lowland west before molting allows molt migrants to escape these conditions of low productivity in late summer. At the same time, a July and August storm system known as the "Mexican Monsoon" brings abundant precipitation to central and western Mexico. Indeed, the tropical deciduous thorn forests of western Mexico are leafless until the monsoon season. With the late summer rains, these forests burst from dormancy with blooms, fruits and new green foliage - creating a cornucopia of insects, nectar and fruits that migrant songbirds can exploit to support the energetic demands of their annual molt.
Thus the "push" of late summer aridity favors songbirds of the lowland west moving away from their breeding grounds to molt, and the "pull" of the Mexican Monsoon favors them moving south for their annual molt. After completing their molt, most of these birds move further south for the remainder of the winter, presumably because the monsoon region becomes dry and unproductive in mid winter. By the end of December the coastal band of tropical deciduous forest in west Mexico is again mostly leafless.