Hybridization in the Purple Grackle, Quiscalus Quiscula

Abstract

W HEN natural hybridization in aniAV 7mals is discussed, the example of Quiscalus quiscula is usually mentioned. Many examples of such hybridization are known, but the grackles are unusual in that there is a considerable zone in which the intermediates are found in abundance without either of the end forms. Chapman (1936, 1940, and other papers) accounted for the present distribution of the species on the basis of a post-glacial reunion between populations isolated in Florida on the one hand and southern Texas and northeastern Mexico on the other. This theory was based on studies of the geographical distribution of the brilliant and often iridescent interference colors of museum specimens of male birds presumed to be breeding. The differences between the forms are seen chiefly in the feathers of the back. In the bronzed grackle, Quiscalus quiscula versicolor, these feathers are uniformly brassy-bronze to olivebronze. The range of this form extends north to the limit of trees and west to the Rocky Mountains from a line running from southwestern Louisiana north through Mississippi and east through southern Tennessee to the Appalachians, then northeast across the mountains, reaching the Atlantic coast in the region of New York City. The other well-defined end form is the Florida grackle, Q. q. quiscula, whose dorsal feathers are dark green on the exposed terminal part, bordered basally by an iridescent band. This form is found south of a line running from southern Louisiana across southern Mississippi and Alabama, central Georgia, and western South and North Carolina, reaching the coast in the region of the Virginia capes. The specimens of these two forms examined in this work came from the localities shown in Map 1 (p. 168). Overlapping these ranges and occupying the region in between are intermediates completely connecting the end forms. Those which have brassy-green backs and merge into the bronze-backed versicolor are called Quiscalus quiscula ridgwayi, and the purplebacked intermediates which blend with quiscula are called Q. q. stonei. Map 2 (p. 169) shows the distribution of specimens of these forms and their intermediates with each other and with the end forms. The color of the head and neck of this species varies from purplish-brassy through blue to brassy-green, but this variation is largely independent of that of the back color. The whole range of head colors has been observed in association with each of the back colors, but green and blue heads are not commonly associated with the dark green back. In Florida the head is always purple, but blue and green heads begin to appear north of that state. The relative frequencies of the head colors do not differ appreciably from stonei through ridgwayi to versicolor. Chapman proposed that during one of the glacial ages, probably the most recent one, the Wisconsin, the two end forms became differentiated from each other while in their refuges. In the northern part of the range of quiscula, the greenbacked Florida form, a purple-backed form, stonei, became differentiated. As the climate warmed up, stonei met and interbred with versicolor along a line from Louisiana to Massachusetts, producing a brassy-green-backed hybrid form, ridgwayi. Chapman suggested that the purple