Great Plains Hybrids The grasslands and prairies of the Great Plains once presented an impenetrable barrier to avian dwellers of forests, woodland edges, and thickets in the East and West. All that changed with the advent of European-style agriculture and the planting of trees. Fingers and islands of deciduous forest along rivers and streams, on farms, and in towns began to reach out across the plains. They created suitable habitats for range expansion that affected 14 pairs of closely related, ecologically similar but geographically separated species. These closely related pairs of species had evolved in isolation from each other, and bringing them into contact resulted in hybridization (interbreeding) between members of most pairs of congeners (members of the same genus), as shown in the following table: Range Expansion Affecting 14 Pairs of Closely Related, Ecologically Similar but Geographically Separated Species. Eastern Form Western Form Hybrids Studied? E. Screech-Owl W. Screech-Owl Rare No Yellow-shafted Flicker Red-shafted Flicker Common Yes Red-bellied Woodpecker Golden-fronted Woodpecker Rare No Great Crested Flycatcher Ash-throated Flycatcher Unknown No E. Wood-Pewee W Wood-Pewee Unknown No Blue Jay Steller's Jay Rare No? Carolina Chickadee Black-capped Chickadee Rare No Tufted Titmouse Black-crested Titmouse Common Yes E. Bluebird Mountain Bluebird Very rare* No Rose-breasted Grosbeak Black-headed Grosbeak Various ** Indigo Bunting Lazuli Bunting Common Yes Rufous-sided Towhee Spotted Towhee Common Yes E. Meadowlark W. Meadowlark Rare Yes Baltimore Oriole Bullock's Oriole Common Yes * Only a single hybrid known ** Common only along Platte River in central Nebraska; hybrids rare or unknown elsewhere; well studied in only a few locales Although this "spread of agriculture and planting of trees" scenario appears likely for at least some of these species (e.g., the jays), extensive field studies and better understanding of the dynamics of hybridization over the past 20 years have led to challenges of this view. The stability of hybrid zones for several species pairs has led some ornithologists to conclude that contact between them predates the arrival of European agriculture. In fact, Audubon recorded a mixed pair of flickers and their brood near the present Montana-North Dakota border, an area where hybrid flickers still predominate. Across the range of locales where they overlap, only seven of the 14 species pairs have been adequately studied. Based on these studies, members of four of the hybridizing pairs previously treated as separate species have been combined into single species (Northern Flicker, Tufted Titmouse, Rufous-sided Towhee, Northern Oriole). The orioles and buntings are particularly well studied and provide some interesting examples of hybrid zone dynamics. It appears that the Baltimore Oriole form is extending its range westward and replacing the Bullock's Oriole form both in the Canadian prairies and along the Platte River in Nebraska and Colorado. Along the Platte River, the geographic center of the hybrid zone between the two orioles has shifted 200 km westward over the past two decades. In contrast, the Bullock's appears to be extending eastward across southern Kansas and replacing the Baltimore at a similar pace, with the hybrid zone center shifting by 100 km eastward over the past decade. In spite of these shifts, the width of the hybrid zone has not increased. For the most part, there is little indication of assortative mating (the preferential mating of each oriole with individuals of the same form) or of selection against the hybrids. Although an estimated one-third of Indigo and Lazuli Buntings hybridize in areas of contact where they are equally abundant, hybrids appear to have reduced viability, putting them at a selective disadvantage relative to pure Indigos or Lazulis. For this reason, the two buntings are, unlike the orioles, considered sufficiently distinct to be classified as separate species. The range of the Indigo has expanded westward by 200 km over a 15-year period in northern Nebraska and in western Kansas. In areas of regular contact, song switching between the species and interspecific territoriality both occur. The extent of hybridization between members of the other seven pairs is poorly known for most areas of contact. The amateur as well as professional ornithologist can help clarify the relationships between these closely related species by carefully noting the composition of breeding pairs in these areas and their reproductive success. SEE: Hybridization; Species and Speciation; Natural Selection; Superspecies; Sibling Species; Taxonomy and Nomenclature. Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.