Tawny Owl Strix aluco response to call-broadcasting and implications for survey design

Capsule: The use of call-broadcasting significantly increases the number of Tawny Owls Strix aluco
detected in winter point counts, but requires careful survey design to avoid introducing potential
sources of bias into population estimates.
Aims: To examine Tawny Owl response to call-broadcasting to aid survey design in national
monitoring efforts.
Methods: A nocturnal survey was undertaken at 36 survey points over three nights in winter in
Thetford Forest, England. Each survey consisted of four consecutive five-minute segments: a
passive count, followed by three counts with the use of call-broadcasting.
Results: Few (4%) Tawny Owls were recorded during passive surveys, whereas the greatest
response was during the first and second call-broadcast segments (49% and 36%, respectively).
New detections declined to 11% in the final segment. Response was fastest at dusk, although
time of night did not significantly affect the number of individuals detected. Male owls
accounted for 79% of detections.
Conclusion: Our results show that ten minutes of call-broadcast surveying will detect 85% of
responsive Tawny Owls, thus vastly improving detection compared to passive listening alone.
However, simultaneous counts of geographically separated detections should be used to provide
a minimum count and reduce potential double-counting of mobile individuals.