Research Article Burrowing Owl Mortality in the Altamont Pass Wind Resource Area K. SHAWN SMALLWOOD, 1 3108 Finch Street, Davis, CA 95616, USA CARL G. THELANDER, BioResource Consultants, P.O. Box 1539, Ojai, CA 93024, USA MICHAEL L. MORRISON, Department of Wildlife and Fisheries Sciences, Texas A&M University, College Station, TX 77843-2258, USA LOURDES M. RUGGE, 2 California State University, Sacramento, Department of Biological Sciences, 6000 J Street, Sacramento, CA 95819, USA ABSTRACT We estimated wind turbines in the Altamont Pass Wind Resource Area (APWRA), California, USA, kill .100 burrowing owls (Athene cunicularia hypugaea) annually, or about the same number likely nesting in the APWRA. Turbine-caused mortality was up to 12 times greater in areas of rodent control, where flights close to the rotor plane were disproportionately more common and fatalities twice as frequent as expected. Mortality was highest during January through March. Burrowing owls flew within 50 m of turbines about 10 times longer than expected, and they flew close to wind turbines disproportionately longer within the sparsest turbine fields, by turbines on tubular towers, at the edges of gaps in the turbine row, in canyons, and at lower elevations. They perched, flew close to operating turbine blades, and collided disproportionately more often at turbines with the most cattle dung within 20 m, with the highest densities of ground squirrel (Spermophilus beecheyi) burrow systems within 15 m, and with burrowing owl burrows located within 90 m of turbines. A model of relative collision threat predicted 29% of the 4,074 turbines in our sample to be more dangerous, and these killed 71% of the burrowing owls in our sample. This model can help select the most dangerous turbines for shutdown or relocation. All turbines in the APWRA could be shut down and blades locked during winter, when 35% of the burrowing owls were killed but only 14% of the annual electricity was generated. Terminating rodent control and installing flight diverters at the ends of turbine rows might also reduce burrowing owl mortality, as might replacing turbines with new-generation turbines mounted on taller towers. ( JOURNAL OF WILDLIFE MANAGEMENT 71(5):1513–1524; 2007) DOI: 10.2193/2006-307 KEY WORDS Altamont Pass, Athene cunicularia hypugaea, behavior, burrowing owl, fatalities, mitigation, mortality, nest density, wind turbines. The western burrowing owl (Athene cunicularia hypugaea; hereafter owl) in California, USA, has declined in number and spatial distribution (DeSante et al. 1996, Klute et al. 2003). Nesting pairs occur individually or in small groups scattered among remnant habitat patches across their former geographic range, and previously known populations were extirpated in recent years (e.g., Johnson 1997). Owls residing in the Altamont Pass were omitted from the last statewide assessment of burrowing owls (DeSante et al. 1996). Researchers studying avian collisions with wind turbines observed many burrowing owls in the Altamont Pass Wind Resource Area (APWRA), as well as numerous owl carcasses under wind turbines (Rugge 2001; Thelander et al. 2003; Smallwood and Thelander 2004, 2005). Owls appeared so numerous in the APWRA that we concluded the population there must be regionally significant, and there appeared to be sufficiently high mortality to wonder whether the APWRA functions as an ecological sink. Recent data suggested owls collide with wind turbines more often where owl burrows are more numerous near turbines (Smallwood et al. 2001). Smallwood et al. (2001) found the number of owl burrows (i.e., all burrows showing fresh signs of use, such as whitewash, pellets, decorations, or flushed owls) increased with the number of ground squirrel (Spermophilus beecheyi) burrow systems within 55 m of turbines. Owl nest sites close to wind turbines may expose owls to a greater threat of collision with wind turbines. Therefore, wind turbines located on the landscape away from owl burrows may reduce turbine-caused owl mortality. Our research objectives in the APWRA, as they related to burrowing owl, were to 1) compare human-caused mortality to the predicted habitat capacity of owls in the APWRA, 2) identify key patterns of owl mortality and behavior in the APWRA, and 3) relate management practices to owl behavior and mortality patterns, and suggest mitigation measures to reduce turbine-caused owl fatalities in the APWRA. STUDY AREA The APWRA was in central California’s eastern Alameda and southeastern Contra Costa counties, including about 16,450 ha of annual grassland. Our study area ranged from 78 m to 470 m above mean sea level, was composed of hills, ridges, and valleys, and included stock ponds, small seasonal ponds, and marshes. Most ridges were oriented northwest to southeast and were bisected by seasonal streams. Vegetation was predominantly nonnative annual grass- land, including soft chess (Bromus hordeaceus), rip-gut brome (Bromus diandrus), foxtail barley (Hordeum murinum lepor- inum), Italian rye grass (Lolium multiflorum), and wild oats (Avena fatua). Common forbs included black mustard (Brassica nigra), fiddle-neck (Amsinckia menziesii interme- dia), chick lupine (Lupinus microcarpus var. densiflorus), bush lupine (Lupinus albifrons), and wally baskets (Triteleia laxa). Grasses and forbs grew during the rainy months of January through March, then died or went dormant by early June. Physiographic elements of the APWRA included annual 1 E-mail: puma@davis.com 2 Present address: ECORP Consulting, Inc., 2260 Douglas Boulevard, Suite 160, Roseville, CA 95661, USA Smallwood et al.  Burrowing Owls in Altamont Pass 1513