Testing detectability of radio-tracked Tawny Owls using playback broadcast surveys: designing evidence-based surveys
Published: Volume 67.2, July 2020. Pages 355-369.
Owls, like other secretive species, are difficult to detect. During the last few decades, protocols used for surveying owls have revealed the best methods to acquire accurate data about their distribution and abundance. However, these protocols were established by testing the response of owls to playback of broadcast calls, using wild, unknown individuals that were only noticeable when vocalisations were given/detected. Therefore, there is no clear consensus on the best method to survey owls. We tested a protocol to survey Tawny Owls Strix aluco using 20 radio-tracked individuals in two study areas with contrasting owl density. we conducted 58 survey tests during winter 2014-2015. In the two study areas respectively, 61.3% and 70.4% of the target individuals responded, on average, within the first six minutes after the start of call playback. Naïve occupancy estimates were 67.7% and 85.2%, respectively, considering the combined responses of the target individual and its mate. we detected a movement of the target owls from their original position towards the playback source on 72.4% of the sampling occasions in both areas. Tawny Owls approached within 50m of the playback point on only 56.9% of occasions. Mates or neighbours responded to broadcast calls more often in the high-density area than in the low density one. The detection probability of target owls increased more than fourfold when their mates joined in defence of the territory, and also increased fourfold when the target individuals approached the broadcast point and when we increased the playback period from five to fifteen minutes. we recommend two 15-minute periods of call playback per point and year, on dry and calm winter nights, at survey points one kilometre apart.