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University of California Press and CooperOrnithological Society are collaborating with JSTOR to digitize, preserve and extend access to The Condor.Screech Owls (Otus asio) occur widely across the deserts of, the southwestern United States and the northwestern states of Mexico. In these arid regions they range from the oak and pifion belts of the mountains down through the desert scrub habitat of the lowest basins and river valleys. Mesquite brush, even when well scattered and not more than six feet in height, and widely spaced palo verdes, ironwoods and the larger cacti are adequate plant cover for the desert races of this species. Nocturnal foraging for insects and rodents on or near the ground requires but sparse vegetation, and daytime concealment and nest sites are probably the needs that limit these owls to the vicinity of the dense, larger-trunked, even if low, desert shrubs or to the large cacti or streamside cottonwoods in which woodpecker cavities are frequent. In especially favorable terrain, male owls stationed on territories may be spaced less than one hundred yards apart, although the interval is more commonly two hundred to four hundred yards. They are, then, an abundant small raptor, much more abundant, and hence we may say more successful, than their diurnal raptorial counterparts of the same areas, the Sparrow Hawks, Loggerhead Shrikes and Road-runners.The area of concern in our study of Screech Owls lies south of central Nevada and Utah, west of the continental divide in the main, and east of the desert divides of southern California; it extends south through Sonora and Baja California. This area is essentially the lower Colorado River drainage basin and the watersheds of the trough of the Gulf of California: Some adjoining parts of the Great Basin region are involved.In this area and about its margins eight races of Screech Owl are currently recognized. It is not proposed that any more be recognized, although one does require renaming. The object of our study has been, then, to clarify the nature of the differences in the races, to describe the clines in characters running through the complex and their general environmental correlations, and to indicate the points of maximum change in characters which may serve as somewhat arbitrary race boundaries. The races involved in the area are Otus asio cineraceus in the northeast, 0. a. suttoni marginally in the southeast, 0. a. sinaloensis in southern Sonora, O. a. gilmani, herein renamed, in the center of the Colorado trough, 0. a. inyoensis in the Great Basin, O. a. quercinus on the western desert border, and 0. a. cardonensis and 0. a. xantusi in the central and southern parts of the peninsula of Baja California. Sp...