United States Department of Agriculture The California Spotted Owl: Current State of Knowledge cover Forest Pacific Southwest General Technical Report August Service Research Station PSW-GTR-254 2017 In accordance with Federal civil rights law and U.S. Department of Agriculture (USDA) civil rights regulations and policies, the USDA, its Agencies, offices, and employees, and institutions participating in or administering USDA programs are prohibited from discriminating based on race, color, national origin, religion, sex, gender identity (including gender expression), sexual orientation, disability, age, marital status, family/parental status, income derived from a public assistance pro- gram, political beliefs, or reprisal or retaliation for prior civil rights activity, in any pro- gram or activity conducted or funded by USDA (not all bases apply to all programs). Remedies and complaint filing deadlines vary by program or incident. 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Submit your completed form or letter to USDA by: (1) mail: U.S. Department of Agriculture, Office of the Assistant Secretary for Civil Rights, 1400 Independence Avenue, SW, Washington, D.C. 20250-9410; (2) fax: (202) 690-7442; or (3) email: [email protected]. USDA is an equal opportunity provider, employer, and lender. The use of trade or firm names in this publication is for reader information and does not imply endorsement by the U.S. Department of Agriculture of any product or service. Technical Editors R.J. Gutiérrez is a professor and Gordon Gullion Endowed Chair Emeritus, Univer- sity of Minnesota, 2003 Upper Buford Cir., St. Paul, MN 55108; Patricia N. Manley is a research program manager, Conservation of Biodiversity Program, U.S. Depart- ment of Agriculture, Forest Service, Pacific Southwest Research Station, 2480 Carson Rd., Placerville, CA 95667; and Peter A. Stine is a biogeographer and retired director of Partnerships and Collaboration, U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station, John Muir Institute for the Environment, Univer- sity of California–Davis, 1 Shields Ave., Davis, CA 95616. Authors Brandon M. Collins is a research fire scientist, U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station and the University of California–Berkeley, 140 Mulford Hall, Berkeley, CA 94720. R.J. Gutiérrez is a professor and Gordon Gullion Endowed Chair Emeritus, University of Minnesota, 2003 Upper Buford Cir., St. Paul, MN 55108. John J. Keane is a research wildlife ecologist, U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station, 1731 Research Park Dr., Davis, CA 95618. Maggi Kelly is a geographer, Cooperative Extension specialist and professor, Department of Environmental Science, Policy, and Management, University of California–Berkeley, 130 Mulford Hall, Berkeley, CA 94720. Patricia N. Manley is a research program manager, Conservation of Biodiversity Program, U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station, 2480 Carson Rd., Placerville, CA 95667. Malcolm P. North is a research forest ecologist, U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station, 1731 Research Park Dr., Davis, CA 95618. M. Zachariah Peery is an associate professor, Department of Forest and Wildlife Ecology, University of Wisconsin, 1630 Linden Dr., Madison, WI 53706. Susan L. Roberts is a wildlife ecologist and private consultant, P.O. Box 2163, Wawona, CA 95389. Mark W. Schwartz is a professor and director of the John Muir Institute of the Environment, University of California–Davis, 1 Shields Ave., Davis, CA 95616. Peter A. Stine is a biogeographer and retired director of Partnerships and Collabo- ration, U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station, John Muir Institute for the Environment, 1 Shields Ave., University of California–Davis, CA 95616. Douglas J. Tempel is a postdoctoral research associate, Department of Forest and Wildlife Ecology, University of Wisconsin, 1630 Linden Dr., Madison, WI 53706. i The California Spotted Owl: Current State of Knowledge R.J. Gutiérrez, Patricia N. Manley, and Peter A. Stine, Technical Editors U.S. Department of Agriculture, Forest Service Pacific Southwest Research Station Albany, California General Technical Report PSW-GTR-254 August 2017 ii Abstract Gutiérrez, R.J.; Manley, Patricia N.; Stine, Peter A., tech. eds. 2017. The California spotted owl: current state of knowledge. Gen. Tech. Rep. PSW- GTR-254. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station. 294 p. This conservation assessment represents a comprehensive review by scientists of the current scientific knowledge about the ecology, habitat use, population dynam- ics, and current threats to the viability of the California spotted owl (Strix occiden- talis occidentalis). It is based primarily on peer-reviewed published information with an emphasis on new scientific information that has emerged since the first technical assessment for the California spotted owl (CASPO) was conducted in 1992. Substantial new information and insights exist for owls inhabiting the Sierra Nevada, but much less exists for populations inhabiting the central and southern California parts of its range. Spotted owls are habitat specialists that are strongly associated with mature forests that are multistoried or complex in structure, and have high canopy cover, and an abundance of large trees and large coarse woody debris. Most California spotted owl habitat is concentrated in mid-elevation forests of the Sierra Nevada, which consist primarily of ponderosa pine (Pinus ponderosa Lawson and C. Lawson), mixed-conifer, white fir (Abies concolor (Gord. & Glend.) Lindl. ex Hildebr.), and mixed-evergreen forest types. Currently, there are about ~2 million ha (~5 million ac) of suitable habitat in the Sierra Nevada, with 75 per- cent occurring on national forests. These habitat conditions have been demonstrated to have a strong positive association with key vital rates (e.g., occupancy, adult survival, reproductive success), which drive population persistence. All studies published since CASPO have demonstrated that owl populations on national forests in the Sierra Nevada have declined over the past 20 years. A preponderance of evidence suggests that the past century’s combination of timber harvest and fire suppression has resulted in forests that have a considerably higher density of trees but a reduced density of large-diameter trees and logs, a greater density of shade-tolerant fire-sensitive tree species, and an increase in forest fuels. These conditions have resulted in reduced habitat quality, increased habitat frag- mentation, and increased risk of high-severity fire in the Sierra Nevada. Climate change is projected to have significant effects on Sierra Nevada forests, including exacerbating the risk and impacts from high-severity fires, which in turn is likely to affect spotted owl habitat and populations. The specter of additional threats in the iii form of competition from the newly invading barred owl (Strix varia) and environ- mental contaminants, as well as the continuing dearth of information on central and southern California populations, further raises concerns about the fate of California spotted owl populations. Maintenance of a viable population of spotted owls in the Sierra Nevada and throughout its range will depend on effective, long-term owl conservation practices embedded in an overall management strategy aimed at restoring resilient forest structure, composition, and function, including reducing the risk of large-scale high-severity fires while reducing the risk of habitat loss to the owls. Keywords: California spotted owl, Strix occidentalis occidentalis, conservation assessment, national forest, Sierra Nevada, forest resilience, USDA Forest Service, viability, wildfire. iv Contents 1 Chapter 1: Introduction Peter A. Stine and Patricia N. Manley 1 Regulatory Context 2 1992 California Spotted Owl Technical Assessment 3 Lessons Learned From the Northern Spotted Owl 4 Expanding Challenges in Spotted Owl Conservation 6 This Conservation Assessment 8 Information Sources Consulted 8 The Process and Product 9 Literature Cited 11 Chapter 2: The Biology of the California Spotted Owl R.J. Gutiérrez, Douglas J. Tempel, and M. Zachariah Peery 11 Introduction 12 Taxonomy 14 Ecology 15 Behavior 19 Reproductive Ecology 23 Habitat Associations 28 Diet 30 Cause-Specific Mortality 31 Chapter Summary 34 Literature Cited 47 Acknowledgments 49 Chapter 3: California Spotted Owl Habitat Characteristics and Use Susan L. Roberts 49 Introduction 49 Habitat Characteristics 49 Nest and Nest Tree Characteristics 50 Nest Stand Characteristics