The North Platte River valley (elev. ca 3300 ft/990 m) in Garden and Keith counties, Nebraska, has an avifauna of 305 species, the richest known north of Texas in the Great Plains. More than 25 years of observations, mist-netting, banding, and breeding-bird surveys by the authors and others have revealed 104 breeding, 17 probably breeding, and 184 transient, casual, and accidental species. Hybridization of eastern with western species is evidenced by intermediates between Rose-breasted and Black-headed grosbeaks, Indigo and Lazuli buntings, Eastern and Spotted towhees, Baltimore and Bullock\u27s orioles, and yellow- and red-shafted morphs of Northern Flicker; Eastern and Western wood-pewees potentially hybridize because both breed in the ar...