REPRODUCTIVE ISOLATING MECHANISMS IN THE BLUE‐WINGED WARBLER – GOLDEN‐WINGED WARBLER COMPLEX

Abstract

Speciation phenomena have received considerable attention from geneticists, taxonomists, and evolutionary theorists. A primary impasse to our knowledge has been the paucity of field studies employing direct observations of species in some critical stage of this process. The birds chosen for this study, Blue-winged Warblers (Vermivora pinus) and Golden-winged Warblers (V. chrysoptera), are at a critical point; the parental phenotypes continue to occur although fertile hybrids are produced. We deal with the reproductive relationships of the two species and hybrids. Data from field studies and the literature lead to conclusions about pairing mechanisms, the success of hybrids vs. parental types, and possible factors promoting reproductive isolation as well as those increasing interbreeding. This species complex is ideal for investigations of speciation for several reasons: (1) the zone of sympatry and hybridization is extensive, (2) their history of contact is reasonably well-documented, (3) there are many accounts in the literature of mixed pairs in different areas of sympatry, and (4) field observations are not difficult early in the season since the birds are conspicuous and have rather small territories. The Blue-wing, formerly the more southern species, has expanded its range northward in the last hundred years and is now extensively sympatric with the Golden-wing (Short, 1963; Berger, 1958). When birds intermediate in plumage were discovered in the late 1800's, there was much speculation concerning their origin. Finally Faxon (1913) carefully followed a mixed pair and found that progeny resembling the Brewster's Warbler, one of the intermediate types, resulted from this mating. Unfortunately, the great interest in these species then waned, since the origin of intermediate specimens was no longer a mystery. Only recently have ornithologists begun detailed investigations. Parkes (1951) proposed an interpretation of the inheritance of plumage color and pattern supported by observations of the progeny of mixed pairings. Berger (1958) outlined historical changes and the ecological status of the two species and hybrids in the Great Lakes states. More recently, Short (1962, 1963), studying the two species and hybrids in several areas of sympatry by detailed analyses of specimens, showed that introgression has taken place. The present study is based mainly on investigations in two areas, but is far from definitive because the situation is complex and ever-changing throughout the area of sympatry. There is great need for more intensive studies in many different areas and over a long period of time; only then will all of the complex processes become clear. This paper is one of a series on song, courtship, ecology and territory in Blue-winged Warblers, Golden-winged Warblers and their hybrids (Ficken and Ficken, 1966 in press a, b, c.)