Monitoring northern spotted owls on federal lands in Marin County, California: 2009 annual report

Press, David; Merkle, William; Jensen, Heather

This report provides a summary of results from the 2009 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 subspecies 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 30 known spotted owl territories (“sites”) were randomly selected and monitored using standardized methods during the 2009 breeding season. We used an interim sample design in 2009 while in the process of revising and formalizing our sample design for our long-term northern spotted owl monitoring protocol.Spotted owl pair occupancy in 2009 was the second highest reported, and fecundity was above average for the program’s 11 years of monitoring. Pairs of spotted owls occupied 27 of the 30 (90%) long-term monitoring sites. Of the 26 females with known reproductive status, 19 females (73%) attempted nesting. With two nest failures, the remaining 17 nests yielded a confirmed total of 27 young. A total of seven non-nesting females were confirmed at the 30 sites. In 2009, the fecundity estimate of 0.52 (SE 0.09) was above the average fecundity of 0.40 (SD 0.22) measured at monitoring sites from 1999 to 2009. The percentage of pairs that attempted nesting (73%) in 2009 was above a 10-year average of 61%. A good nesting year, possibly influenced by favorable weather conditions and abundant prey, may have attributed to the high rate of pair occupancy at sites monitored in 2009. In addition, our random selection of sites for monitoring resulted in a lower percentage of sites that are impacted by barred owls than in the previous two years of monitoring. Our monitoring history has shown that in the presences of barred owls, northern spotted owls are difficult to detect and pair occupancy rate is decreased. By randomly selecting a lower percentage of sites influenced by barred owls for our 2009 monitoring, we likely increased our chances of recording northern spotted owl pairs at the sites we monitored.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: 2009 annual report

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