Fine-scale movement data reveal surface foraging and nocturnal flight activity in the endangered Bermuda petrel

Abstract

Foraging behavior plays a fundamental role in animal fitness and population dynamics, particularly in marine ecosystems where seabirds like petrels (small Procellariiformes) showcase a diverse array of foraging strategies finely adapted to the pelagic environment. The extent and remote nature of their foraging grounds makes direct observation of foraging behaviour impractical, thereby requiring the use of remote tracking technologies. 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. Time-depth-recorders provided new insights into petrel feeding techniques suggesting that the meso-bathypelagic prey targeted by petrels must be available in the very upper layer of the water surface, given their very limited diving activity (99.99% of dives had 0.1 m of depth). Accelerometer data revealed 3 flying- and 3 water-related behaviours. Flying behaviours reflected the expected dynamic soaring flight strategy of procellariforms; individuals spent more than three-quarters of their time in flight with flying-non-flapping being the most common behaviour under all conditions. The behaviour classified as "flying-intensive" was infrequently observed but 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 thought to reflect scavenging and prey seizing. Flying-related behaviours increased with negative sun elevation values, highlighting greater flight activity during night compared to the day, while water behaviours were more common during the day. While some of our findings may require further validation to confirm their relevance to foraging behaviour, our work offers new and valuable insights to consider when assessing the extent and nature of offshore anthropogenic-related risks faced by petrels.