Morphological Variation and Evolution in Some North American Orioles

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Abstract
Rising, James D. (Univ. Toronto, Dept. Zoology, Ramsay Wright Zoological Laboratories, Toronto 5, Ontario, Canada) 1970. Morphological variation and evolution in some North American orioles. Syst. Zool., 19:315–351 [oriole: Icterus galbula; Icterus bullockii; Icterus abeillei; hybridization; hybrid zone; introgression; species concepts; step dines].—Patterns of morphological variation of three taxa of orioles, Icterus galbula (Baltimore oriole), I. bullockii (Bullock oriole) and I. abeillei (black-backed oriole) (Icteridae; Aves) are discussed. Baltimore orioles breed in eastern North America, Bullock orioles in the western half of that Continent, and black-backed orioles on the Mexican Plateau. Distributional overlap is minimal. In most parts of their ranges individuals of either sex can be easily identified as to type by any of an array of features of size, color pattern, or vocalization. However, where the distributions of the morphs appear to abut, considerable phenetic variation is manifest. Special attention is here given to one such region of apparent abutment, the central Great Plains, where there is an abrupt east-to-west transition of Baltimore-morph to Bullock-morph and where some populations consist of orioles that are obviously phenetically intermediate. It has been suggested that the two forms are incipient bio-species, hybridizing in a geographically narrow region where secondary contacts have recently occurred. It is pointed out that step-clines of the sort manifest among Plains populations of orioles can arise by means other than secondary contact, and in the present context it seems unnecessarily speculative to postulate such rejunctions. On the basis of logical, distributional, and historical evidence, there is no reason to believe that the human activities of the past 150 years have had any fundamental effect on the distribution of the orioles in the Plains, and neither the incidence of phenetically intermediate individuals nor the width of the zone that essentially describes their distribution seems to be increasing. Rather, all evidence suggests that there is considerable stability among these populations, such that the patterns of distribution of the various morphs are coincident with some of those of the climatic environment. Doubtless many agencies of selection are acting in concert to dynamically maintain the “steps” in the oriole morph-clines in the Plains. Among these, the demonstrable difference between Baltimore and Bullock orioles' high temperature tolerance must be of considerable significance. Also, the differences between the Baltimore- and Bullockmorphs are so great that birds that disperse across the “hybrid zone” are probably selected against as being “unrecognizable” or as “odd birds.” It is impossible, within the described limits of non-phenetic concepts of the species, to designate as separate species or not these groups of orioles without relying upon a priori assumptions that are untenable or teleological, or without inviting a posteriori circularity. I have preferred nomenclatural separation for the groups for the sake of convenience, but such a decision is arbitrary.

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