Analysing the intermittent flapping flight of a Manx Shearwater, Puffinus puffinus, and its sporadic use of a wave-meandering wing-sailing flight strategy

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Flights of Manx Shearwaters over the Irish Sea were investigated using GPS (n=6) and simultaneous high sample rate triaxial accelerometry (n=1). This pelagic species executes flight through intermittent bursts of flapping flight interspersed with gliding phases while meandering low over the waves. To facilitate the analysis and interpretation of body-mounted accelerometry in these challenging circumstances we introduce a combined time and frequency domain technique allowing accurate separation of flapping from gliding, measurement of wing-beat frequency and determination of flapping duty cycle. Considerable fluctuations in cycle period and time-averaged flapping duty cycle were found. Our approach offered high temporal precision, which was crucial as half the flapping bursts were briefer than 0.8s and half the cycle times shorter than 2.55s. Flapping duty cycles exceeding 38% were likeliest for short range flights and ascending flights. At higher duty cycles, cycle time decreased and wing-beat frequency and amplitude was only moderately elevated. Near-continuous flapping was only observed during steep ascents and strong headwinds. During a long-range foraging flight with good GPS coverage duty cycles between 7% and 63% were observed. We posit that flapping was modulated in order to maintain a steady airspeed in somewhat variable wind and wave conditions as part of a complex wave-meandering wing-sailing flight strategy that was often effective in reducing locomotion costs. Periods of very low duty cycle flight appear to have benefited from instantaneous crosswinds exceeding 10ms−1 with an estimated three-fold reduction in biomechanical power. Accelerometry offers a very practical tool for studying flight performance and the methods herein described can be readily adapted to other species that intermittently beat their wings.