Rapid Evolution in Orioles of the Genus Icterus

Abstract

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 (1964) 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 hybridization, 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 1973 the A.O.U. Check-list Committee accepted the recommendation by Sibley and Short (1964) that the western forms (bullockii) be treated as subspecies of I. 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