It has long been recognized that the Great Plains represent a major transition zone in the distribution patterns of North American birds; field guides traditionally have treated the 100° W. longitude meridian as a convenient dividing line between eastern and western faunas. Furthermore, this line rather neatly bisects the political subdivisions of the Great Plains, namely the "plains states" extending from North Dakota southward through South Dakota, Nebraska, Oklahoma, and Texas. Of these, Texas is the least typical, its climate and fauna is strongly influenced by the Gulf Coast on the east and the Chihuahuan desert on the west. As a result of its size and ecological diversity Texas supports the largest array of breeding bird species of any state in the nation. Thus, in the present analysis it was decided to eliminate from consideration all but the northwestern "panhandle" of Texas, which consists of the grassland-dominated "staked plains." Further, to facilitate the faunistic analysis, limiting lines of latitude and longitude were selected that not only encompassed all five other states mentioned, but also parts of several adjoining ones. After some deliberation, it was decided to define the coverage of the analysis as extending from the U.S.-Canadian border (49° N. latitude) southward to 34° N. latitude in Texas, following this line eastward until it intersects with the Texas-Oklahoma boundary, and continuing eastwardly along the boundary to the eastern limit of Oklahoma. The western limit was defined as the 104° W. longitude meridian, which essentially conforms with the western boundaries of the Dakotas and the Nebraska "panhandle", and continues southward through the eastern portion of Colorado and New Mexico. The eastern limit was selected as the 95° W. longitude meridian, which includes the prairie areas of western Minnesota, the western edge of Iowa, and a small part of extreme northwestern Missouri. Where this line intersects with the Missouri River along the Missouri-Kansas border, the coverage was continued eastwardly to include the extreme eastern portions of Kansas and Oklahoma (Fig. 1).
A minimum of 324 species have recently bred or currently breed in the area under consideration. Five of these are introduced species, and were excluded from the analysis, and another seven species are either extinct or have been extirpated from the region. Of the 312 remaining species, about 260 are "regular" breeders. Thus well over 50 percent of America's continental breeding bird fauna is included within the region's geographic limits, in spite of the fact that it is less than a fifth of the total area of the U.S.A. excluding Alaska. A listing of all the species included in this analysis, together with their geographic and ecologic affinities, may be found in Tables 1 to 5, and a numerical summary of these listing appears in Table 6.
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