Geographic Variation of the Screech Owls of the Deserts of Western North America
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.
... f 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 diffqences 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, 0. (I. gilmani, herein renamed, in the' center of the Colorado trough, 0. a. inyoensis in the Great Basin, 0. a. quercinus on the western desert border, and 0. a. cardonensis and 0. a. xantusi in the central and southern harts of the peninsula of Baja California. Special effort has been exerted for a period of twenty years to collect western Screech Owls in order better to understand the wide range of racial variation in this species. The impetus for this work ca.me from the realization that these owls could be taken in numbers much greater than formerly by nocturnal hunting employing the whistled imitation of the principal trilling note of the western races to induce aggressive calling, and often approach, on the part of the owl. Use of this technique was begun by Loye Miller and independently by Edouard C. Jacot and in time it was adopted by a number of other field students. Among those whose nocturnal activity has also contributed extensively to the collections now before us are In 1944 van Rossem and Loye Miller began a series of special trips into the deserts of California, Arizona, and Sonora in order to collect Screech Owls and pursue a revisionary study of the group. Van Rossem assembled loan material at the Dickey Collections in Los Angeles with the result that by 1949 there was present there a total of 365 adult specimens pertaining to the deserts and their peripheries, almost half of which had been taken by Loye Miller and van Rossem in recent years. The untimely death in 1949 of van Rossem, who was to have prepared the report, left on record only a partial set of _ measurements and occasional indications of his views on identifications on the measurement sheets. Some of these were interpretable from verbal comments he had made to the present authors. Utilization of the unique assemblage of Screech Owls seemed not only mandatory but challenging, and accordingly Alden Miller began periodic study of it in 1949. Use was made also of 73 specimens at the Museum of Vertebrate Zoology additional to those previously borrowed by van Rossem. Thus a total of 438 birds bearing on problems in the area has been employed. For the loan of specimens we are greatly indebted to the following persons and institutions: The TJniversity of Arizona, the Museum of Northern Arizona, Joseph Brauner, the Chicago Natural History Museum, Robert G. Hannum, Lyndon L. Hargrave, Lawrence M. Huey, Randolph Jenks, the Los Angeles Museum, the Museum of Zoology of the University of Michigan, Gale Monson, Allan R. Phillips, the late Max M. Peet, the San Diego Society of Natural History, William J. Sheffler, the United States National Museum (Biol. Surv. Coil.), and the ' University of Utah. The large group of specimens available to us has served to develop an appreciation of the great range of individual variation in Screech Owls, both in color and dimensions, which early workers with small series could scarcely gain. As a consequence certain names based on small samples and slight individual variations can now be assigned to synonymy with some confidence. It is thought that naming of such variants and of intergrading stages badly obscures and distorts the main pattern of geographic variation. The sex ratio in the collections is influenced by the greater readiness of males to respond aggressively to imitated calls. The most thorough comparison of geographic samples can, therefore, be made in this sex. However, in some areas, such as the Tucson district and' the Cape district of Baja California, many Screech Owls were taken by climbing to nest and roost holes before nocturnal hunting was much in vogue. Consequently females are better represented or even predominate slightly in the samples from these areas. GEOGRAPHICALLY VARIABLE CHARACTERS Color and pattern.-In dealing with the extremely variegated and complex color patterns of owls, it is difficult to make comparisons and focus attention on comparable parts in different specimens and series. Mass effects of contrast are often readily seen but are trodblesome to. analyze. On the dorsal surface the principal geographic variables are: (1) ' the width and intensity of the dark shaft streaks, which are always more prominent on the head than on the back and which especially on the head increasingly dominate the pattern as seasonal wear proceeds; (2 j the hue and shade of the grays and browns that chiefly in mottled form make up the principal background color of the back; and (3) the hue and tint of the light tawny or essentially white areas, especially where they are prominent in the neck region. On the face and ventral surfaces the variables are: (1) the amount and purity of the white which predominates in the face, especially at the margins of the ocular-auditory ' disc; (2 ) the width of the black shaft streaks of the ventral surface, which are always two to three times wider on the breast than on the flanks; those of the same section of the flanks, for example, may vary geographically from 0.5 to 2.5 mm. in width; (3) the July, 1951 VARIATION IN DESERT SCREECH OWLS 163 164 THE CONDOR Vol. 53 * Geographic groupings used for measurements (for outlines of areas see corresponding numbers on map, fig. 2) .-I, southeastern Arizona and southwestern New Mexico, largely the Chiricahua