RARITY OF SPOTTED OWLS IN SOUTHERN CALIFORNIA

Abstract

The California spotted owl (Strix occidentalis occidentalis), an older forest species of conservation concern, has declined in many historically occupied forests due to habitat loss from severe wildfire and other environmental changes. Although population trends have been well documented in the Sierra Nevada mountain range in the western United States, less is known about population status at the range boundary in Southern California, a region experiencing rapid environmental change. Therefore, we conducted extensive spotted owl nighttime surveys during the 2022 breeding season in the San Gabriel Mountains of the San Bernardino and Angeles national forests, the Sierra Pelona Mountains of the Angeles National Forest, and the Los Padres National Forest. These surveys included historically occupied territories; spatially extensive, low-intensity surveys outside of these historical territories; and surveys in regions with potentially high-quality owl habitats. We compared remotely sensed data of vegetation conditions and disturbance between currently occupied and currently vacant territories, and at currently vacant territories during the period of historical occupancy and after the loss of territorial owls, to assess potential causes of population change. We made 1,913 visits to call points, surveying an area of 1,100 km2, yet we located only eight currently occupied territories. Out of 13 historically occupied territories, only 3 (23%) remained occupied. Currently, spotted owls were more likely to occupy territories with a greater basal area of deciduous trees. Furthermore, currently vacant territories had greater levels of drought-related tree mortality after the period of detection of the last owls. Our findings suggest that habitat scarcity, exacerbated by drought-induced tree mortality, declines in oaks, and possibly warming temperatures, may be contributing to spotted owl population declines in Southern California, although the small number of occupied territories in the region limited inferences. Our results align with other recent work indicating this species' rarity and decline in Southern California, with functional but declining populations in the San Bernardino Mountains.