Northern Spotted Owls in Marin County, California: 2005 Annual Report

Jensen, Heather; Adams, Dawn; Merkle, William

This report details the results of the ninth year of the Northern Spotted Owl (Strix occidentalis caurina) Monitoring Program in Marin County, California. The goal of the monitoring study is to estimate trends in Spotted Owl occupancy rates and productivity on federal, state, county, and water district lands within Marin County. The intended audience of this report includes partners in our joint monitoring program as well as appropriate agencies at the county, state, and federal levels. The report provides an overview of the Marin County Northern Spotted Owl Monitoring Program and summarizes the results of the data collected during the 2005 field season. The report does not include comprehensive descriptions of methodologies or demographic data analysis. During the 2005 breeding season, Marin County’s survey teams made 289 visits to a total of 46 Spotted Owl territories designated as long term monitoring sites. Of the 46 long-term monitoring sites, a subset of 30 sites was randomly selected to determine reproductive status. At the remaining 16 of the 46 long-term monitoring sites, researchers confirmed Spotted Owl occupancy status, at a minimum, including age and band identification of any owls that were observed. An additional 15 sites were monitored based on resource management needs of the land management agencies involved in the project. For this report, only information pertaining to the 46 long-term monitoring sites was included, with the exception of the analyses of nesting habitat, nest structures, and age class of paired owls. The 2005 breeding season was an average reproductive year for Spotted Owls locally. Pairs of Spotted Owls occupied 83% or 38 of the 46 long-term monitoring sites. At the 30 sites monitored to reproductive status in 2005, a total of 25 sites had pair occupancy. Of the 26 females with known reproductive status, 18 females (69%) attempted nesting. Thirteen of the 18 females successfully nested yielding a confirmed total of 21 young. Twenty percent of nesting attempts in 2005 resulted in nest failure, as defined in the Marin Modified Protocol. A total of four non-nesting females were confirmed at the 30 sites monitored to reproductive status. Fecundity for 2005 in Marin County was 0.46 (s.e. r0.096) which reflects the average fecundity of 0.46 (s.e. r0.078) for the years 1998-2005. A total of 26 nests were located in 2005. Twenty-two nests were new and the four nest structures (three platforms and one cavity) were reused. Of the 195 unique nests documented from 1997 to 2005, 19 (9%) were cavities and 176 (91%) were a type of platform nest. This ratio is the opposite of owl nests in older forests where 80-90% of the nests are in cavities, but closely resembles the ratio in other parts of the range where forests are younger. In 2004, the banding portion of the study was terminated due to logistical constraints and limited returns from previous efforts. Of the 110 Northern Spotted Owls banded before 2004, 31 (14 females and 17 males) were resighted in 2005 including three that were resighted in new locations. Since 2000, a total of 11 owls (8 juveniles and 3 adults) whose identity can be positively determined have been resighted at new territories. The average distance moved was 4.4 km (n=3) for adults and 7.4 km (n=8) for juveniles. This Spotted Owl population, at the southern limit of the sub-species’ range faces unique threats due to the close proximity of a growing human population. Threats include disturbance by park visitors and birding enthusiasts, development along wildland/urban interfaces, impacts of Sudden Oak Death, the continued Barred Owl (Strix Varia) detections in the county, and the spread of West Nile Virus throughout in Marin County. We continue to use our modified protocol that minimizes the use of “mousing,” while allowing determination of indicators of population health.