Body mass of streaked shearwaters affects wingbeat during ascending and sea-surface flight

Streaked shearwaters (Calonectris leucomelas) return to their breeding colonies, which are established in elevated locations on islands, by increasing their flight altitude immediately before landing. We quantitatively analysed the characteristics of this energetically demanding ascending flight and examined how body mass influences flapping behaviour using high temporal resolution biologging data from eight individuals. We compared the proportion of time spent flapping, the period of dorsal-ventral axis acceleration cycle (the inverse of wingbeat frequency) and acceleration amplitude between ascending and sea-surface flight. The proportion of time spent flapping was considerably larger during ascending flight (94.6±6.5%) than during sea-surface flight (40.8±13.0%). The mean dorsal-ventral acceleration cycle was markedly shorter (0.233±0.008 s versus 0.247±0.008 s) and the mean amplitude was larger (8.85±0.82 m s-2 versus 7.20±0.49 m s-2) during ascending flight than during sea-surface flight. During ascent, birds increased wingbeat frequency, accompanied by greater body acceleration amplitude, indicating increased mechanical power output required for climbing flight. The increases in wingbeat frequency and amplitude were larger for heavier birds when transitioning from sea-surface to ascending flight, revealing that body mass strongly influenced flapping performance under high energy demand scenarios. Individual variation in body mass strongly affected the biomechanics and energy requirements of flight, providing insights into size-related trade-offs in the flight behaviour of island-breeding seabirds.