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. Received 25 January 1982, resubmitted 6 May 1982, accepted 18 April 1983. THE ranges of Baltimore and Bullock's orioles (Icterus galbula and I. bullockii) • meet in the Great Plains. In regions of contact, from southern Alberta to central Texas, variability in plumage color and pattern exists, and in the most variable populations the majority of individuals (70-90%) are variously intermediate between the two nominal forms. Throughout this region there is an abrupt step-cline in plumage pattern, from Baltimore-like orioles in the east to Bullock's-like orioles in the west (Sibley and Short 1964, Rising 1970, Corbin and Sibley 1977). On the prairies, orioles most commonly are • At present, the A.O. U. (1982) combines these two as the Northern Oriole, Icterus galbula (this presumably includes a third well-marked form from Mexico, Abeille's Oriole). It has been argued that the status of these orioles is uncertain (Rising 1970, 1973; Corbin and Sibley 1977), so, for the sake of clarity, I use the well-established and unambiguous English common names "Baltimore" and "Bullock's" orioles in this paper. For the Great Plains orioles, appropriate trinominals are I. g. galbula and I. g. bullockii, respectively. 885 found in open riparian woodlands along the trans-Plains rivers, which run essentially southeastwards from the Rocky Mountains to the Mississippi River. At different times, the oriole plumage step-cline has been described along several of these trans-Plains rivers. Sutton (1938) collected "hybrid" orioles from along the Canadian River in western Oklahoma in the 1930's; Sibley and Short (1964) described the step-cline in the plumages of male orioles collected from along the Platte and Niobrara rivers in Nebraska and Colorado in the 1950's; Rising (1970) described the clines both for males and females taken from along the Republican, Smoky Hill, Arkansas, and Cimarron rivers in Kansas; and Anderson (1971) described the orioles from along tributaries of the Missouri River in South Dakota in the 1960's. As early as 1908, Bent described "hybrid" orioles from the Cypress Hills region in the southern Canadian prairies, and Rising (1973) described additional hybrids from Montana, Alberta, and Saskatchewan. The north to south correspondence of the step-cline in male plumages from along the different rivers is indirect evidence that this is a stable step-cline, like those described beThe Auk 100: 885-897. October 1983 886 J.D. RISINC [Auk, Vol. 100 tween morphologically and geographically distinctive populations of Rufous-naped Wrens ( Campylorhynchus rufinucha; Selander 1964, 1965), Carrion Crows (Corvus carone; Mayr 1963), Common Grackles (Quiscalus quiscula; Yang and Selander 1968), and house mice (Mus musculus; Hunt and Selander 1973). A step-cline can be stable through time if there is gene flow among populations and selection favors one morph or genotype on one side of the step and another on the other side whether or not intermediate or heterozygous individuals are favored at intermediate sites (in the region of the step) (Endlet 1977, Slatkin 1978). If intermediate individuals were selected against, the step-cline would disappear. If intermediate individuals were selectively equivalent (i.e. no selection) or superior in all regions, the steps would become obscure through time. Thus, if temporally repeated collections show a flattening of step-clines for characters that vary geographically, we can infer that those characteristics are selectively neutral and that there is no "hybrid" advantage. On the other hand, a stable step-cline indicates selective maintenance. Nevertheless, in practice it can be difficult to differentiate between stable and transient clines, because, unless samples are taken at closely adjacent sites along a cline over several generations, a cline that is in fact transient may appear to be stable (Endlet 1977). If a step-cline shifts in geographic position without flattening, however, it is highly probable that selection is maintaining the step. Although the differences in plumage pattern between Baltimore and Bullock's orioles are of no obvious adaptive significance, some of the physiological responses of Bullock's Orioles would seem to give them a selective edge over Baltimore Orioles in hot, dry areas (Rising 1969), and, presumably, Baltimore Orioles have an adaptive edge in roesic conditions. Rising (1970) noted that the plumage step was correlated with a zone of climatic transition in the Plains and argued that the step-cline between Baltimore and Bullock's orioles was stable. Corbin and Sibley (1977) describe changes in the plumages of orioles at several sites along the Platte River in Nebraska and Colorado that were examined first in the mid-1950's. They found that Baltimore-like orioles had extended their range westward along the Platte. At Big Springs, in western Nebraska, a mixed popuBig Springs, Nebraska (A)