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Rapid Evolution in Orioles of the Genus Icterus

1977; Oxford University Press; Volume: 79; Issue: 3 Linguagem: Inglês

10.2307/1368011

1938-5129

Kendall W. Corbin, Charles G. Sibley,

Ecology and Vegetation Dynamics Studies

Hybridizationinvolving orioles of the genus Icterus has been extensive throughout the Great Plains of North America, from southern North Dakota to north central Texas.The first specimens to be recognized as hybrids were described by Sutton ( 1938), and the zone of hybridization has been studied by Sibley and Short ( 1964), Rising ( 1968, 1969, 1970, 1973), Anderson ( 1971)) and Misra and Short (1974).The specimens collected during the 1950' s by Sibley and Short (4964) indicated that the parental forms were behaving biologically as subspecies.As clarified by Short ( 1969), the parental types were allopatric and separated by a hybrid zone.At that time there was no zone of overlap and hyhriclixation, within which both parental types existed in sympatry and in a combined frequency greater than 5% of the total population.That is, there was no apparent reinforcement of isolating mechanisms.In I973 the A.O.U.Check-list Committee accepted the recommendation by Sibley and Short (1964) that the western forms (bdockii) be treated as subspecies of 1. galbula.This decision was based primarily on birds collected along the Platte River, extending from Greeley, Colorado to Blair, Nebraska.However, Rising (1968, 1970) believed that the "Bullock' s Oriole" and "Baltimore Oriole" should not be treated as conspecific until the extent of introgression was better known.cussion of these data are presented in this paper.MATERIALS AND METHODS Two collections of adult orioles provide the data for this paper.The first consists of 271 birds collected during the 1950' s by Sibley and Short (1964) from the following localities: Nebraska: Blair, Schuyler, Gothenburg, Sutherland, and Big Springs; Colorado: Crook, Fort Morgan, and Greeley.The data for these birds are presented in Table 5 and Figure 2 of Sibley and Short (1964) and in Tables 1 and2 of Misra and Short ( 1974): The second group of birds includes 231 adult males collected during the breeding seasons of 1970, 1971 and 1974 (these specimens are now in the collection of the Peabody Museum of Natural History, Yale University).The localities sampled are given in Figure 1 and Table 1.The