Detection of Barn Owls (Tyto alba) During Nighttime Call-Broadcast Surveys

Abstract

Evaluating raptor populations is challenging because of their low population densities and secretive breeding behavior. Barn Owls (Tyto alba) are particularly difficult to monitor given their nocturnal nature and irregular vocalization patterns. In an effort to help develop more effective survey protocols for locating breeding Barn Owls and for monitoring Barn Owl populations, we examined factors influencing Barn Owl detections and responses to broadcast surveys. We conducted 288 surveys at 33 different nest locations in 2013 and 2014 in Illinois, USA, and examined the influence of using call broadcasts, breeding stage, day of year, cloud cover, moon phase, time of night, wind speed, ambient noise, temperature, and the number of Barn Owls in proximity on detection rates. We detected Barn Owls on 11.2% of passive surveys and 49.4% of broadcast surveys; 97.6% of the detections during broadcast surveys involved individuals flying within 20 m of observers. Detections were more common earlier in the breeding season and in areas where density of known Barn Owl nests was greater, but these variables affected detections from broadcast and passive surveys similarly; other variables did not have meaningful associations with Barn Owl detections. These results suggest that broadcast surveys may be an effective approach for identifying areas where Barn Owls are breeding or for monitoring population trends.