Winter ecology of northern Saw- Whet Owls (Aegolius acadicus) in the Sierra Nevada foothills of California
Northern Saw-Whet Owls (NSWO, Aegolius acadicus), one of the smallest owls in North America, migrate in large numbers every fall from higher latitudes and elevation to overwinter at both lower latitude and elevation. While extensive efforts have been devoted to understanding their migration patterns in the eastern US, little is known regarding NSWO migration patterns or winter habitat preferences in the west. The goals of this study were to determine wintering destination of local migrant NSWO, winter habitat, and diet patterns for the first time in California using the Big Chico Creek Ecological Reserve (BCCER) as a study area. NSWO were captured at BCCER during fall migration and fitted with 2.5-gram radio-transmitters. During the fall/winter of 2010-2012, a total of 19 NSWO were tagged and 112 different roost sites were described. Over 80% of diurnal roosts were between two species: toyon (Heteromeles arbutifolia) and canyon live oak (Quercus chrysolepis). Paired random sites were also sampled 50 meters from roost sites to determine roosting preferences. Shrub canopy cover was found to be significantly higher at owl roosts compared to random. A diet analysis of 77 pellets collected between ten owls showed NSWO took few prey species, primarily consuming western harvest mice (Reithrodontomys megalotis), comprising 48% of 103 prey items. Voles (Microtus spp.) and deermice (Peromyscus spp.) made up the remainder of the diet. Though NSWO are a common species, little is known about their general habits in the west; therefore results from this study contribute to our understanding of this species.