Distribution and abundance of red tree voles in Oregon based on occurrence in pellets of northern spotted owls

Red tree voles are one of the least understood small mammals in the Pacific Northwest, because they live in the forest canopy and are difficult to sample using conventional trapping methods. We examined the distribution and relative abundance of tree voles in different regions of Oregon based on their occurrence in diets of northern spotted owls. We identified the skeletal remains of 2,954 red tree voles in regurgitated pellets collected from 1,118 different spotted owl territories. Tree voles were found in the diet at 486 territories. They were most common in the diet in the central and south coastal regions, where average owl diets included 13% and 18% tree voles. They were absent from owl diets on the east slope of the Cascades and in most of the area east of Grants Pass and south of the Rogue River. Our data were sparse from the northern Coast Ranges and northern Cascades, but suggested comparatively low numbers of voles in those regions. The proportion of tree voles in the diet was negatively correlated with elevation in the Cascades, where tree voles were common in the diet at elevations 1,220 m. The highest elevations at which tree voles were detected in owl diets were 1,324 m in the Cascades and 1,390 m in the Klamath Mountains. On average, we estimated that nesting pairs of spotted owls captured 54 tree voles per year in western Oregon, but there was large variation among and within regions. Although our data indicate that tree voles are widespread in Oregon, and fairly common in some regions, it is likely that tree vole populations have declined in areas where logging, fire, and human development have produced landscapes dominated by young forests.