Fine-scale movement data of the endangered Bermuda petrel highlights surface foraging and greater nocturnal flight activity

Abstract

Foraging behavior plays a fundamental role in animal fitness and population dynamics, particularly in marine ecosystems where seabirds like petrels showcase a diverse array of foraging strategies finely adapted to the pelagic environment, but rarely studied. However, the extent and remote nature of their foraging grounds makes direct observation of foraging behaviour impractical, thereby requiring the use of remote tracking technology. We deployed miniaturized multi-sensor biologgers and collected fine scale movement data to investigate the at-sea behaviours of the Bermuda petrel Pterodroma cahow, a poorly studied and highly threatened gadfly petrel, specialised on mesopelagic prey. GPS-tracking data revealed extensive foraging trips, in consistent directions, over remote oceanic regions. Accelerometer-data analysis highlighted three flying-related and three water-related behaviours. Flying behaviour reflected the expected dynamic soaring flight strategy of procellariforms; individuals spent more than three-quarters of their time in flight with non-flapping flight being the most common behaviour under all conditions. A third and less frequent flight behaviour: the intensive flight, could indicate 'aerial dipping', a characteristic foraging technique of Pterodroma species. The remaining time was spent in three water behaviours: active, inactive, and intensive, with the latter being less common but likely reflecting scavenging and prey seizing. All behaviours but intensive water and flight had a diel activity pattern. Flight-related behaviours increased with negative sun elevation values while water-related behaviours showed an opposite trend, highlighting greater flight activity during nighttime. Finally, time-depth-recorders complemented the results suggesting that the meso-bathypelagic prey targeted by petrels must be available in the very upper layer of the water surface, given the very limited diving capacity of petrels. While some of our findings may require further validation to confirm their relevance to foraging behaviour, our work offers new insights to take into consideration when assessing the extent and nature of offshore anthropogenic-related risks faced by petrels.