The Stability of the Oriole Hybrid Zone in Western Kansas

Abstract

In the Great Plains of North America a number of species of birds that breed in woodland edge in the east are replaced by western counterparts (Rising 1983a). In many instances (Colaptes, Passerina, Pheucticus, Pipilo, and Icterus) the counterpart congeners apparently hybridize, often rather commonly, where their ranges are in contact. It is widely postulated that these eastern and western counterpart taxa differentiated in allopatry during the Pleistocene (Mengel 1970), and today are interbreeding following rather recent secondary contact (Rising 1983a). The consequences of this hybridization are not clear, but may be clarified by long-term studies (Selander 1965, Yang and Selander 1968, Rising 1983b, Moore and Buchanan 1985). The eastern Baltimore Oriole (Icterus galbula) and the western Bullock's Oriole (I. bullocki) are among the best studied of these east-west species pairs (Rising 1983a, 1983b). They hybridize in a zone of apparent secondary contact that runs from southwestern Alberta south through northeastern Montana and western North and South Dakota, Nebraska, Kansas, and Oklahoma. In 1965 and 1966 I collected orioles in riparian woodlands (principally cottonwoods [Populus]) from 14 sites along four river systems in western Kansas, the Republican, Smoky Hill, Arkansas, and Cimarron, that run essentially west to east through the oriole hybrid zone (Rising 1970). In 1976 and 1978, I recollected orioles from six of the sites that I had previously collected in the 1960s, along the Cimarron and Smoky Hill rivers, as well as a sample of Bullock's Orioles from along the Cimarron River near Kenton, Oklahoma, on the Oklahoma-New Mexico border (Rising 1983b) to serve as a western reference sample. Here I report on orioles that I collected in 1992 and 1993 from the same sites, shown in Figure 1, that I had collected in the 1970s; in 1992, I also collected a sample of Baltimore Orioles from central (Rice Co.) Kansas, to serve as an eastern reference sample. These three sets of collections, taken over a nearly 30-year span, allow me to directly assess the changes of oriole hybridization in western Kansas. METHODS