Nebraska and its adjoining states represent a nearly unique ornithological situation. Although they are almost entirely grassland, the plains states are variably dissected by river-bottom forests that provide natural passageways for forest-adapted species to enter the plains and sometimes to cross them. Although eastern and western bird faunas tend to be separated by the plains, they often mingle to some degree, resulting in competition and sometimes hybridization. The central Great Plains thus are exceedingly interesting from ecological, evolutionary, and zoogeographic perspectives, and the plains bird fauna is significant both for what is present and for what has been variably excluded.
I first hoped to consider the entire Great Plains as a comprehensive unit, but a survey of the literature rapidly made it apparent that from either a geological or a botanical standpoint the region was far too large to be dealt with easily. I thus began to consider various compromises between my initial comprehensive vision and the restrictive approach of including only one or two states. I felt that the region covered should essentially saddle the 100th meridian-traditionally considered the dividing line between eastern and western bird faunas- and should include as many of the essentially grassland-dominated states as feasible. An excellent recent book on the breeding birds of North Dakota (Stewart 1975) provided a kind of northern "anchor" for the work, and recent and comprehensive books on the birds of Texas (Oberholser 1974) and Oklahoma (Sutton 1967) did the same for the southern portion of the region. It was thus necessary to establish only eastern and western limits, which was done by choosing those lines of longitude that included the maximum of grassland habitats and the minimum of forest or montane communities.
Available at: http://works.bepress.com/paul_johnsgard/270/