The Progress of Oriole Hybridization in Kansas

Specimens of Northern ("Baltimore" and "Bullock's") orioles collected at six sites in western Kansas in 1976 and 1978 are compared with specimens collected at the same sites in the mid-1960's. No changes in the location or size of the step-cline ("hybrid zone") between the two taxa are indicated by comparisons of the plumage features of the male specimens; the distribution of phenotypes of birds from sites where both Baltimore and Bullock's orioles occur is not bimodal, and, therefore, there is no indication of selection against intermediate birds. Female Bullock's-like orioles, however, are found farther east along the Cimarron River in southwestern Kansas than they were in the 1960's. The correlations between the amount of precipitation at a locality in western Kansas and the phenotype of oriole that occurs there are high (about 0.90). This suggests that precipitation per se is a factor that determines the distributional limits of these birds relative to each other. There are no recent changes, however, in the average climatic conditions in western Kansas. Perhaps the orioles present at a site in any given season are a reflection of the conditions there the previous season or two. The analyses of size variation among samples collected in the 1970's show an east-west cline that is more-or-less congruent with that found for plumage features; at any given site, the variability in size is positively correlated with the variability in plumage. Where a diversity of oriole phenotypes occurs, there is no tendency for females to associate with males of the "same type" (i.e. Bullock's with Bullock's or vice versa); this suggests that assortative mating does not occur.