Combining bio-logging, stable isotopes and DNA metabarcoding to reveal the foraging ecology and diet of the Endangered Bermuda petrel Pterodroma cahow

The foraging range of central-place predators is limited by spatiotemporal and energetic constraints. Gadfly petrels Pterodroma spp. are seabirds well adapted to travel vast distances in oceans with sustained wind conditions, although our understanding of their foraging ecology in these heterogeneous and highly dynamic environments is still limited. We studied the foraging behaviour, habitat use, ecological niche and diet of the Endangered Bermuda petrel P. cahow, endemic to the western North Atlantic. We used GPS loggers to track foraging trips during incubation and early chick-rearing in 2019 and 2022, and employed DNA metabarcoding coupled with stable isotope analyses to reveal dietary habits. Our analyses showed that petrels travelled over a vast area of the western North Atlantic while foraging over deep, pelagic waters. Specifically, in the early chick-rearing phase, they reduced their foraging range and time spent at sea compared to incubation. Foraging locations were associated with varying sets of environmental variables between breeding phases, including mesoscale oceanographic features, distance to the colony and wind speed. Petrels also showed narrow isotopic niches, and the ranges of delta 15N and delta 13C values suggested consistency in trophic habits. Finally, we found high taxonomic diversity in the diet, including exclusively meso-bathypelagic fishes and cephalopods. Our results contribute critical new knowledge on Bermuda petrel foraging-behaviour plasticity, a feature that can help predict how a small population of an endangered species may respond to climate-related changes in wind regimes and oceanic processes expected in the North Atlantic Ocean.