Flight style and time-activity budgets of the smallest petrels

Procellariforms are the most oceanic among birds, regularly embarking on the longest journeys in the animal kingdom to find food over an apparently featureless sea surface. To minimize energy expenditure, many species harness wind energy through dynamic soaring, extracting kinetic energy from the wind shear. The smallest members of this order, storm petrels, have functional traits that prevent this type of locomotion, and are predicted to rely on flapping flight despite their high motility. However, theoretical predictions have never been validated and their flight strategy and activity budgets are unclear. We hypothesized that, as the benefits of dynamic soaring are out of reach, these birds should rely on gliding to some extent to sustain their long-ranging movements and save energy. To test our hypothesis we used, for the first time, miniaturized Inertial Measurement Units on one of the world's smallest seabirds, the European storm petrel (Hydrobates pelagicus). We demonstrate that these small seafarers spend 78% of the time flying during their foraging trips, with wingbeat frequencies up to 15 Hz. During transiting, they flap wings at high frequency (mean±SD: 8.8±0.8 Hz) for 91% of the time, gliding sporadically and for instants only (mean±SD: 0.11± 0.17 sec). Flight activity was high during night and early morning, while they rest on the sea in the central hours of the day. Overall, our results reveal a peculiar locomotory strategy among procellariforms and raise questions about how they can energetically sustain foraging trips spanning hundreds of kilometres.