Hybridization in the Grackle Quiscalus quiscula in Louisiana

Abstract

The northern form of the common grackle, Q. q. versicolor, inhabiting pine forest and mixed pine-hardwood forest, ranges south in Louisiana to the latitude of Baton Rouge, Opelousas, and De Ridder, where it interbreeds freely in a narrow zone with the southern form, Q. q. quiscula, inhabiting cypress-tupelogum swamp forest and coastal marshes, to produce viable and fertile hybrids. Analysis of geographical variation in 4 color characters in 637 males from 21 sample areas in Louisiana and the application of discriminant function analysis to form a weighted hybrid index demonstrate that a complete transition in color characters between Q. q. versicolor and Q. q. quiscula occurs between 30[degree] and 31[degree] N latitude. Populations of the northern form are only slightly introgressed with genes from Q. q. quiscula, whereas those of the southern form are heavily introgressed with genes from Q. q. versicolor. Geographical variation in characters of size is smoothly clinal, lacking the step-cline feature seen in characters of color. The position and width of the hybrid zone in Louisiana is related to the pattern of distribution of the major vegetation types. Where the mixed pine-hardwood habitat of Q. q. vesicolor contacts the cypress-tupelogum habitat of Q. q. quiscula, the zone is 15 miles wide, but it widens to 40 miles where the 2 habitat types are separated by areas of disturbed bottomland hardwood forest. A comparison of morphological variation in material taken in the period 1890-1942 with that taken in the period 1962-1965 demonstrates that, since the 1930''s, the zone of hybridization in Louisiana has shifted northward approximately 20 miles. Apparently the northward shift was not accompanied by a change in the width of the zone. In the absence of evidence of the functioning of reproductive isolating mechanisms, the extraordinary narrowness and temporal stability of width of the zone of hybridization can only be explained in terms of the hybrid inferiority hypothesis.