Scheduling Differences of Molt and Migration for Baltimore and Bullock's Orioles Persist in a Common Environment

Abstract

Rohwer and Manning (1990) recently showed that Bullock's Orioles (Icterus galbula bullockii) migrate to the American Southwest prior to undergoing their fall molt. Thus, recently fledged young that are still in full juvenile plumage and adults that are in worn breeding plumage migrate at least as far as the southwestern U.S. and northern Mexico before undergoing their prebasic molt. Although Sealy (1979) reports that some second year male Baltimore Orioles (Icterus galbula galbula) may begin moving southward early in the fall molt, both the first and later prebasic molts of galbula take place on the breeding grounds. Rohwer and Manning (1990) suggest that bullockii migrates to the American Southwest before molting to escape the extremely dry late summer conditions of the coastal lowlands and intermountain west. In contrast, late summer monsoon rains characterize the American Southwest and should produce a food flush that birds could exploit to support their postjuvenile and postbreeding molts. Recently both Lazuli Buntings (Passerina amoena) and western populations of Painted Buntings (P. ciris) have also been shown to move to the American Southwest for their fall molts (Young 1991, Thompson 1991). Because Rohwer and Manning (1990) initiated their study of these orioles to test predictions distinguishing winter versus summer explanations of delayed plumage maturation (Rohwer and Butcher 1988), the specimens they examined largely excluded bullockii taken east of the Rocky Mountains. This left open the possibility that the differences in the fall molt/migration schedules of galbula and bullockii might be under environmental rather than genetic control. Interestingly, Corbin and Sibley (1977) report evidence of a reduced frequency of galbula x bullockii hybrids between the mid-1950s and the early 1970s along the Platte River of eastern Colorado and western Nebraska. Rohwer and Manning (1990) suggest that the dramatic differences in the molt and migration schedules of these orioles could constitute a potent source of selection against hybrids. For this to be true, however, the differences in the scheduling of their fall molts relative to the migration must be under genetic control. To eliminate environmental control we sought to examine all late summer specimens of bullockii and galbula from the Great Plains, a region where they should experience much more similar environmental conditions than they do in their larger allopatric ranges west of the Rocky Mountains and in eastern North America.