Monitoring northern spotted owls on federal lands in Marin County, California: 2014-2015 report

Ellis, Taylor

This report provides a summary of results from the 2014 and 2015 field seasons 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 they have been minimally impacted by barred owl (S. varia) range expansion. 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 39 known spotted owl sites were randomly selected and monitored using standardized methods during the 2014 breeding season. Pairs of spotted owls occupied 34 of these 39 (87%) long-term monitoring sites. Of the 34 territorial females in 2014, 23 females (68%) attempted nesting. Fourteen of the nests were successful and yielded a confirmed total of 23 young. A total of 3 non-nesting females were confirmed at the 39 sites. Nine nest failures were documented in 2014. In 2014, the fecundity estimate of 0.48 (SE 0.10) was above the average fecundity of 0.36 (SE 0.06) measured at monitoring sites from 1999 to 2015. In 2015, we again randomly selected and monitored 39 spotted owl sites. Pairs of spotted owls occupied 31 of the 39 sites (79%). Of the 31 territorial females in 2015, 11 attempted nesting. Eight of the nests were successful and yielded a confirmed total of 9 young. A total of 13 non-nesting females were documented at the 39 sites, and we confirmed 3 nest failures. In 2015, the fecundity estimate of 0.24 (SE 0.09) was below the average fecundity of 0.36 measured at monitoring sites from 1999 to 2015. NPS continued to monitor a resident pair of barred owls at Muir Woods. The pair nested in 2014 and successfully fledged at least two young. The male of this pair, which was fitted with a radio transmitter in 2012, was found dead in March of 2015, and the female was collected in February 2015 as part of a UC Berkeley study of hybridization between barred and spotted owls. Two single barred owls were detected in two other spotted owl sites in 2014, but in 2015 we only detected one other barred owl in a spotted owl site. That owl was collected in November 2015 as part of the hybridization study. As of December 2015, there were no confirmed barred owls in Marin County. 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: 2014-2015 report

Series

Collections