Differences in Timing and Number of Molts for Baltimore and Bullock's Orioles: Implications to Hybrid Fitness and Theories of Delayed Plumage Maturation

Abstract

We compare the molt and migration schedules of Baltimore (Icterus g. galbula) and Bullock's (I. g. bullockii) orioles, describe how bullockii and Black-backed Orioles (I. g. abeillei) in juvenile plumage can be distinguished, and present a simple quantitative character that serves to distinguish the juvenile and basic plumages of bullockii. Galbula undergoes the first and later prebasic molts on the breeding grounds, while bullockii in both juvenile and worn breeding plumage almost certainly migrate to the American southwest prior to undertaking the fall molts. This early migration by bullockii has probably been favored as a mechanism for them to escape the dry late summer conditions that prevail in most of their breeding range and to exploit for molting the food flush caused by the late summer monsoon rains in the southwest. With but one exception, every bullockii specimen taken in late summer or fall south of the breeding range in Mexico was in fresh basic plumage. Thus, the fall migration of bullockii may be interrupted for the prebasic molts. Bullockii almost entirely lacks a prealternate molt in all sex and age classes. The prealternate molt in galbula is fairly extensive in first-year males but limited in females and in older males. First year galbula males initiate this molt in their throat in early winter, perhaps to acquire a badge that helps distinguish them from first-year females, which have light throats, and from those older females that have light throats. Some galbula males that are almost a year old also undergo part of the prealternate molt after their arrival on the breeding grounds. Implications of these observations for theories of subadult plumages are discussed. These differences between galbula and bullockii in the timing of the fall molts relative to the fall migration and in having or not having a prealternate molt may provide strong sources of selection against hybrids. One specimen illustrating some of the potentially deleterious recombinations of molt and migration schedules among hybrids is described in detail.