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An International Periodical Promoting Conservation and Biodiversity Southwestern United States—Mexico—Central America

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An International Periodical Promoting Conservation and Biodiversity Southwestern United States—Mexico—Central America An International Periodical Promoting Conservation and Biodiversity Southwestern United States—Mexico—Central America Una Revista Internacional para Fomentar la Conservación y Biodiversidad El Suroeste de USA—México—Centroamérica DISTRIBUTION AND ABUNDANCE OF WESTERN BURROWING OWLS (ATHENE CUNICULARIA HYPUGAEA) IN SOUTHEASTERN CALIFORNIA ROBERT L. WILKERSON AND RODNEY B. SIEGEL* Institute for Bird Populations, P.O. Box 1346, Point Reyes Station, CA 94956 *Correspondent: [email protected] THE SOUTHWESTERN NATURALIST 56(3):378–384 SEPTEMBER 2011 DISTRIBUTION AND ABUNDANCE OF WESTERN BURROWING OWLS (ATHENE CUNICULARIA HYPUGAEA) IN SOUTHEASTERN CALIFORNIA ROBERT L. WILKERSON AND RODNEY B. SIEGEL* Institute for Bird Populations, P.O. Box 1346, Point Reyes Station, CA 94956 *Correspondent: [email protected] ABSTRACT—During the 2006 and 2007 breeding seasons, we conducted a systematic survey for western burrowing owls (Athene cunicularia hypugaea) across the portions of California’s southeastern deserts that had never been systematically surveyed for the species. We found few or no western burrowing owls in northern and eastern portions of the Mojave Desert or in the Sonoran Desert (excluding Palo Verde Valley). However, there was a substantial concentration of burrowing owls in the western Mojave Desert, which we estimated to contain #560 (SE 5 268) breeding pairs. We also documented 179 breeding pairs along the banks of water-conveyance structures in Palo Verde Valley in the Sonoran Desert region. These two disjunct populations comprise a significant portion of the population of burrowing owls in California. RESUMEN—Durante las e´pocas de reproduccio´n del 2006 y 2007, se realizo´un estudio sistema´tico de tecolotes llaneros occidentales (Athene cunicularia hypugaea) de las zonas de los desiertos del sudeste de California que nunca habı´an sido muestreados sistema´ticamente para esta especie. Encontramos pocos o ningu´n tecolote llanero occidental ni en las partes nortes y orientales del desierto Mojave ni en el desierto Sonora (excluyendo el valle de Palo Verde). Sin embargo, encontramos una concentracio´n notable de tecolotes llaneros en la parte occidental del desierto Mojave, que se estimo´contener ,560 (SE 5 268) parejas reproductoras. Asimismo, documentamos 179 parejas de tecolotes llaneros en las orillas de las estructuras de conduccio´n de agua en el valle Palo Verde del desierto Sonora. Estas dos poblaciones separadas de tecolotes llaneros comprenden una parte significativa de la poblacio´n total de California. The western burrowing owl (Athene cunicularia nants (Haug et al., 1993; Sheffield, 1997b; hypugaea) has declined in recent decades across Gervais and Anthony, 2003). Each of these much of its range (Wedgwood, 1978; James and factors, and potentially others, may be important Ethier, 1989; Sheffield, 1997a; Holroyd et al., in California, which hosts one of the largest 2001; Wellicome and Holroyd, 2001; DeSante et populations of western burrowing owls of any al., 2007), including California, where it is state or Canadian province (Barclay, 2007). classified as a species of special concern (Gervais Excluding the desert and Great Basin regions, et al., 2008; Shuford and Gardali, 2008). Primary DeSante et al. (2007) estimated the breeding causes of the decline likely have included loss of population in California was 9,266 pairs in 1993. grassland and agricultural habitats to urbaniza- Although burrowing owls occupy the vast deserts tion (Trulio and Chromczak, 2007) and conver- of southeastern California (Garrett and Dunn, sion of lands to inhospitable crops, such as 1981), estimates of size of populations for these orchards and vineyards (Gervais et al., 2008). areas based on systematic surveys have not been Populations in Imperial Valley and in some other published. Anecdotal information indicates that areas of the state, where agricultural practices burrowing owls generally are scarce in the permit, thrive at much higher densities than region, particularly in easternmost portions populations in natural grasslands (DeSante et al., (Garrett and Dunn, 1981), and that a substantial 2004). Other suggested causes of decline include concentration occurs along the Colorado River eradication of fossorial mammals (Zarn, 1974; in Palo Verde Valley (Gervais et al., 2008). Holroyd et al., 2001; J. V. Remsen, Jr., in litt.) However, quantitative, survey-based estimates of and exposure to pesticides and other contami- size of populations and knowledge of distribu- September 2011 Wilkerson and Siegel—Western burrowing owls 379 tional patterns are needed for prioritizing ,1,830 m. Except for inclusion of the Kern Plateau, conservation efforts in California (Burkett and boundaries matched those of the western portion of the Jepson area mapped as Mojave Desert by Hickman Johnson, 2007). (1993) and the California Gap Analysis Project (1998). During the breeding seasons of 2006 and East of the Sierra Nevada, the border of Inyo County 2007, as part of a larger California-wide survey defined the northern boundary. Stratification by (Wilkerson and Siegel, 2010), we conducted a elevation in the western Mojave Desert region was the same as in the northern Mojave Desert-eastern Sierra systematic survey of portions of the deserts in Nevada region. southeastern California, which had not been Our eastern Mojave Desert region was limited surveyed previously. We used results of our primarily to the eastern one-half of San Bernardino survey to characterize patterns of distribution County, south of Inyo County to the Nevada-California and abundance throughout the region and to state line. Boundaries match those of the southeastern portion of the Jepson area mapped as Mojave Desert by estimate size of populations. Hickman (1993) and the California Gap Analysis Project (1998). In southeastern San Bernardino Coun- MATERIALS AND METHODS—We divided previously un- ty, from Cadiz Valley eastward, the eastern Mojave surveyed portions of the breeding range of burrowing Desert region shares an irregular zig-zag border with owls in southeastern California into four regions: the Sonoran Desert region to the south. Stratification northern Mojave Desert-eastern Sierra Nevada, western by elevation in the eastern Mojave Desert region was Mojave Desert, eastern Mojave Desert, and Sonoran the same as in the northern Mojave Desert-eastern Desert. We excluded Imperial and Coachella valleys Sierra Nevada region. because they were surveyed previously by DeSante et al. Boundaries of our Sonoran Desert region matched (2007). Following methods used by DeSante et al. the Jepson area mapped as Sonoran Desert by Hick- (2007), we used ArcGIS software to divide the four man (1993) and California Gap Analysis Project regions into 5 by 5-km blocks, oriented and referenced (1998), excluding Coachella and Imperial valleys, according to the Universal Transverse Mercator (UTM) which bisect the region into two disjunct portions. system. Surveying effort was stratified by elevational The minimal land area in the Sonoran Desert region subregion because populational densities of burrowing .1,220 m elevation was rocky and mountainous; owls generally are higher in lowland areas throughout characteristics that made it inhospitable habitat for California than in upland areas (DeSante et al., 2007). burrowing owls. Thus, we did not survey an upland For logistical reasons, we discarded blocks that could subregion in this region; any block with $5% land area not be accessed by roads, and then we stratified ,1,220 m elevation was included in the region. sampling effort among remaining blocks by region After an intensive training session at the beginning and subregion, randomly selecting as many blocks as of each field season, crew members surveyed blocks we believed our field crew could survey within the time using methods developed by DeSante et al. (2007). allotted in each region. We also identified additional Surveyors visually scanned all of the accessible area in historic breeding blocks where burrowing owls had their blocks at least once during morning (dawn to been detected during any year beginning in 1981. 1000 h) or late-afternoon (1600 h to dusk) during 1 Historic breeding blocks were identified by querying May–30 June 2006 and 2007, when breeding burrowing the California Natural Diversity Database (California owls were likely to be feeding nestlings or recently Department of Fish and Game, in litt.) and consulting fledged young. with knowledgeable researchers and birders with local We provided surveyors with 1:24,000-scale topo- expertise. graphic maps with boundaries of blocks and locations Boundaries of our northern Mojave Desert-eastern of burrowing owls known or suspected to have bred Sierra Nevada region corresponded to portions of Inyo anytime beginning in 1981. Surveyors delineated and Mono counties in the Jepson areas mapped as extent of appropriate habitat in their block, used Mojave Desert and eastern Sierra Nevada by Hickman binoculars or spotting scopes to visually scan all areas (1993) and the California Gap Analysis Project (1998), of appropriate habitat, and plotted locations of any along with a small, disjunct, but ecologically similar detections on their maps. Observers could survey area southeast of Topaz Lake. We divided this region habitat on foot, by automobile, or using both methods, into lowland and upland subregions. Any block with but when surveying by automobile they were instructed $5% of land area ,1,220 m elevation was included in to stop at least
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