Molting While En Route During Fall Migration
As a general rule, most of our small songbirds molt their feathers during what is called the prebasic molt in late summer before they head south for the winter. Interestingly, quite a few western bird species, including the Dusky Flycatcher, Lazuli Bunting, and Bullock’s Oriole provide an interesting exception to that rule. These exceptional species leave their breeding grounds before they molt and they then stop over for several weeks in southern Arizona or northern Sonora to molt before continuing farther south for the winter!
Anecdotal observations of birds captured during fall migration had suggested that molt in some species occurred “during” migration, but it wasn’t until scientists began to look systematically at patterns of molt in museum specimens that the extent and significance of this kind of molt became clear. We now realize that the key to this story involves a pulse in food productivity that follows the late-summer monsoon rains that occur in the Southwest. That food resource, in turn, provides a great opportunity for some bird species to capitalize on the food resources available and to molt at a relatively odd place and time of year.
These research results have resulted in a growing interest in finding and conserving molt-migration stopover “hot spots.” The more we look into the needs of migratory songbirds during all phases of their life cycles, the more we realize that management to maintain their populations will require proper land management not only in the places where they breed and winter, but also in stopover locations that they use while en route between their breeding and wintering haunts.