Territory monitoring of Tawny Owls Strix aluco using playback calls is a reliable population monitoring method

Capsule: Territory monitoring using playback of calls is a reliable approach for assessing population trends of Tawny Owls Strix aluco, particularly when human resources are limited or survey areas are difficult to access. Aims: To explore whether response calls of Tawny Owls towards broadcast conspecific and heterospecific male owl playback calls would provide similar estimates of population size and trends over time as nest-box monitoring. Methods: Between 1998 and 2017, Tawny Owls were monitored in a predominantly forested area of central Slovenia. Throughout the year, territories were monitored using a playback protocol comprising silent listening during five minutes before and after ten minutes of broadcasting male Tawny Owl. Seasonal variation in response rate was examined and results from the playback method were compared to data on occupancy rate of nest-boxes. Results: Territory monitoring using playback calls showed a similar direction of population trends as nest-box monitoring but a different population dynamics pattern. The overall response rate in occupied territories to conspecific playback calls at first visits was 70%. This was significantly higher than for heterospecific playback calls and the frequency of spontaneous vocalizations. The response rate to conspecific playback calls when including two visits rose to nearly 90%. There was no difference in response rate between seasons. The average time to respond to conspecific playback calls was five minutes. Conclusions: Compared to nest-box monitoring of breeding pairs, territory monitoring of breeding and non-breeding Tawny Owls using playback provides a robust and cost-effective method for monitoring. We recommend conducting territory monitoring between January and May during the breeding season, with two visits to each site using conspecific playback of territorial male hoot calls using the 5 + 10 + 5 minutes protocol.