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 (Zcterus g. galbula) and Bullock' s (I. g. bullock@ 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 prealtemate molt in all sex and age classes. The prealtemate 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 prealtemate 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 prealtemate 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.

Increasing numbers of bird species result from taxonomic progress, not taxonomic inflation

“…However, taxonomic studies examined for the present study often revealed previously unrecognized patterns of differentiation. Many newly recognized species of birds differed from their close relatives in traits such as habitat preference (Freitag & Robinson 1993), diet (Perrin 2005), moult (Rohwer & Manning 1990), migratory behaviour (Outlaw et al 2003), breeding system (Zimmer & Whittaker 2000), sexual dimorphism (Buckley & Buckley 2004), body size (Clark & Banks 1992) and flight ability (Kennedy & Spencer 2000). Thus, rather than undifferentiated lookalikes, newly recognized species are often morphologically and ecologically distinct, and merit separate treatment in comparative studies.…”

EVOLUTION OF PLUMAGE COLOR IN MALE PIED FLYCATCHERS (FICEDULA HYPOLEUCA): EVIDENCE FOR FEMALE MIMICRY

“…A number of hypotheses have been proposed to explain this pattern, dealing with individual recognition (Rohwer, 1982), delay of sexual maturation (Selander, 1965), delay of reproductive effort (Procter-Gray and Holmes, 1981;Studd and Robertson, 1985a), status signaling (Rohwer et al, 1980;Foster, 1987;Slagsvold and Lifjeld, 1988a), and spring molt constraints (Rohwer and Butcher, 1988;Rohwer and Manning, 1990). According to the female mimicry hypothesis (Rohwer, 1978;Rohwer et al, 1980), a female-like plumage color is adaptive, since it would help a young male to establish a breeding territory in an area already occupied by conspecific males.…”

Landbird Migration in the American West: Recent Progress and Future Research Directions

“…Although some species likely migrate to their winter grounds to molt, many recent studies found that adults of several western North American species leave the breeding grounds in July or August, stop in the "Mexican monsoon region" in the southwestern United States and northwestern Mexico to molt, and resume migration to the winter grounds in southern Mexico and Central America when molt is complete (e.g., Young 1991, Voelker and Rohwer 1998, Rohwer et al 2005. Whereas not a complete list, species that undergo "molt-migration" to the desert southwest include Bullock's Oriole (Icterus bullockii; Rohwer and Manning 1990), Lazuli Bunting (Passerina amoena; Young 1991), western populations of Warbling Vireo (Vireo gilvus; Voelker and Rohwer 1998), and Western Tanager (Piranga ludoviciana; Butler et al 2002). Many other species depart the breeding grounds before molting and are possible candidates for this list based on feather condition and timing of migration documented for birds passing through western banding stations (e.g., Richardson et al 2003, Carlisle et al 2005a).…”