Movements and Habitat Use of Northern Saw-Whet Owls During Fall Migration

Abstract

We used radio telemetry to track the movements and habitat use of close to 100
Northern Saw-whet Owls (Aegolius acadicus) as they traveled through the Bitterroot Valley during fall migration in 2014 and 2015. We hypothesized that owls would travel south through the Bitterroot River floodplain. Instead, we failed to detect a signal from 19 owls the day after release, suggesting they traveled a minimum of six miles to the east or west into either the Sapphire or Bitterroot Mountains, out of range of our telemetry search. Most of the other owls traveled along the valley periphery, using forested foothills. Most owls exhibited stopover behavior, staying in the same general area for several days between movements. Our greatest nightly distance moved was 40 miles and our greatest distance tracked was 60 miles from the release site. Most owls tracked on the Bitterroot River floodplain roosted high in tall ponderosa pines, in areas with a low density of small trees but a high density of medium-large trees, saplings, and shrubs. We documented one communal roost containing at least three individuals. We only saw half of the owls tracked to an individual tree or shrub; the remaining were too well hidden to detect visually. We did not find pellets and rarely observed whitewash below roosts. These results suggest that methods relying on passive observation to detect owls and/or roost sites likely miss most roost sites, at least during migration.