Monitoring northern spotted owls on federal lands in Marin County, California: 2008 annual report

Jensen, Heather; Merkle, William; Press, David

This report provides a summary of results from the 2008 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). The goals of our northern spotted owl monitoring program are to estimate trends in spotted owl occupancy and fecundity within the NPS legislative boundary in Marin County. A total of 25 known spotted owl territories (“sites”) were randomly selected and monitored using standardized methods during the 2008 breeding season. An additional six sites were monitored based on resource management needs of the land management agencies involved in the project. Spotted owl fecundity in 2008 was the highest reported, while pair occupancy was at its lowest level over the program’s 10 years of monitoring. Five of the 25 historic spotted owl territories were unoccupied in 2008. Pairs of spotted owls occupied 60% or 15 of the 25 long-term monitoring sites. Of the 13 females with known reproductive status, 10 females (77%) attempted nesting. All 10 females successfully nested, yielding a confirmed total of 18 young. A total of three non-nesting females were confirmed at the 25 sites. In 2008, the fecundity estimate of 0.69 (SE 0.12) was well above the average fecundity of 0.39 (SD 0.23) measured at monitoring sites from 1999 to 2008. The decline in spotted owl occupancy may be explained by an expanding presence of barred owls, a competing species, within the study area. In 2008, spotted owls were secretive and difficult to detect at several territories and four spotted owl nests went undetected until fledglings were observed. This imperfect detection of spotted owls may indicate the suppression of spotted owl vocalizations or displacement of spotted owls from their territories as a result of barred owl residency. At least three adult barred owls are known to have established territories on federal lands in Marin County (a pair and a single male). In 2008, barred owls responded to calling surveys from four historic spotted owl territories, all where barred owls have been detected in years prior. Barred owls were detected on a total of eight spotted owl surveys. Only one of the eight surveys also detected a spotted owl. Four of the five spotted owl sites determined to be unoccupied were within three kilometers of a resident male barred owl. For the second consecutive year, we found evidence of barred owl reproduction at Muir Woods producing at least two fledglings. 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 should be initiated. 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: 2008 annual report

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