Assessing patterns of barn owl (Tyto alba) occupancy from call broadcast surveys

3 October 2018

Regan, Tempe; Belthoff, James; McClure, Christopher

Owing to habitat loss, changes in farming practices, urbanization, and high mortality through vehicle collisions, barn owls (Tyto alba) are a species of conservation concern in portions of their range. This species can be secretive and difficult to survey, particularly away from breeding sites, so factors related to barn owl occurrence often remain unknown. We conducted nighttime broadcast surveys for barn owls during the early- and post-breeding seasons and used an occupancy modeling framework to understand how factors related to landcover, landscape features, and human development related to occupancy in southern Idaho, USA. We also assessed the effectiveness of using broadcasts of conspecific vocalizations to improve owl detection. Barn owls were detected during 52 of 666 point counts and at 37 of 222 locations in the early-breeding season and 50 of 198 point counts and 31 of 66 locations in the post-breeding season. The probability of detecting barn owls was 0.32 ± 0.06 (SE) and 0.45 ± 0.07 (SE) during the early- and post-breeding seasons, respectively. Based on analysis within 1-km buffers surrounding point-count locations, occupancy in the early-breeding season increased with percentage of crop coverage and presence of trees and decreased with background noise. Post-breeding season occupancy increased with stream length and decreased with area of development and distance from the Snake River, a major geologic feature that likely provided roost sites in its canyon walls and riparian woodlands and a dispersal corridor for juveniles. Broadcast of barn owl vocalizations increased detection probability as much as nine times. Thus, incorporating call broadcast into future barn owl surveys should help investigators reduce false conclusions of absence. Ultimately, understanding factors influencing occupancy of barn owls will facilitate effective conservation, especially in light of population pressures related to factors such as roadway mortality and loss of nesting sites with increased urbanization.