Using dual-sex calls improves the playback census method for a nocturnal burrow-nesting seabird, the Manx Shearwater Puffinus puffinus. Issue 2 (3rd April 2017) - Perkins, Allan J.
ABSTRACT: Capsule: Playback with dual-sex calls increases the response rate of Manx Shearwaters Puffinus puffinus by 40–60% and reduces its daily variability, which would improve the precision of population estimates. Aims: To test whether playing male and female calls would elicit more responses to playback than male-only calls, reduce daily variability and the length of response rate calibration trials, and give more precise estimates. We also measured response times to playback and assessed the reliability of visual signs of occupancy at burrow entrances. Methods: Responses to four playback call-types (male-only and three variants of dual-sex calls) were compared in repeated trials at two colonies (Ramsey and Rum, UK) during May–June 2014. Results: Dual-sex calls gave higher response rates with lower variance than male-only calls, because females frequently replied to female calls but rarely to male calls. In simulated 3–5-day calibration trials, response rates and correction factors were up to 50% more precise with dual-sex calls. Visual signs of burrow occupancy were unreliable. Conclusion: Playback for Manx Shearwaters should use a 25 seconds recording of male and female calls intermixed, with 10 seconds listening time for delayed responses. Census-specific calibration trials are essential for accurate estimates of daily response rates.