Monitoring northern spotted owls on federal lands in Marin County, California: 2013 Report

Ellis, Taylor; Harrigan, Kaitlin

This report provides a summary of results from the 2013 field season of the National Park Service’s (NPS) northern spotted owl (Strix occidentalis caurina) monitoring program in Marin County, California. The northern spotted owl has been listed as a federally threatened species since 1990. The Marin County population of spotted owls is of interest because of its isolation from other populations, high density and fecundity, and because only recently has this population been affected by the expansion of barred owls (S. varia). There are 110 known spotted owl territories in Marin County, many of which are on or near NPS lands. The goals of our northern spotted owl monitoring program are to estimate trends in spotted owl occupancy and fecundity within the NPS legislative boundaries in Marin County. A total of 40 known spotted owl sites were randomly selected and monitored using standardized methods during the 2013 breeding season. One site was dropped from the study early in the season due to an inability to access the site on private land, giving a final total of 39 sites. Pairs of spotted owls occupied 32 of these 39 (82%) long-term monitoring sites. Of the 34 territorial females in 2013, only 6 females (18%) attempted nesting. Five of the nests were successful and yielded a confirmed total of 7 young. A total of 23 non-nesting females were confirmed at the 39 sites. One nest failure was documented in 2013. In 2013, the fecundity estimate of 0.13 (SE ?0.06) was well below the average fecundity of 0.36 (SE ?0.06) measured at monitoring sites from 1999 to 2013. A barred owl was detected at one spotted owl territory in 2013. NPS staff also monitored a pair of barred owls that is resident at Muir Woods. The pair nested in 2013 and successfully fledged two young. NPS staff and volunteers continued to monitor the adult male barred owl at Muir Woods that was fitted with a radio transmitter in 2012 to better understand barred owl home range size and habitat use. We recommend continued annual monitoring of the spotted owl population, and continuing to share information and work with land managers and county officials to reduce potential adverse impacts of projects on spotted owls. Research focused specifically on barred owls and their impacts on spotted owls in Marin County should be continued. In addition, studies investigating the effects of Sudden Oak Death on spotted owls also are needed. NPS should continue to provide outreach materials related to spotted owl awareness and recovery.

Monitoring northern spotted owls on federal lands in Marin County, California: 2013 Report

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