Revealing the foraging movements and diet of the White-faced Storm Petrel Pelagodroma marina in the NE Atlantic

The White-faced Storm Petrel (WFSP) Pelagodroma marina has a widespread distribution, although virtually nothing is known about their feeding ecology and distributions at-sea. To describe their foraging areas, a total of 77 birds were equipped with 1 g-GPS loggers on Selvagem Grande, Madeira, Portugal (30° 09′ N, 15° 52′ W), during the 2018 and 2019 breeding seasons. We also assessed the diet of WFSP by analysing 17 faecal samples from chicks and 1 regurgitation from an adult using DNA metabarcoding techniques. Additionally, we collected body feathers from ten WFSP chicks to determine mercury concentration. WFSP fed mainly in deep oceanic waters, travelling up to 400 km from the colony, and did not concentrate in any well-defined, population-level foraging hotspots. Some individuals foraged along the edge of the shelf, near the African coast and the Canary Islands, especially during chick rearing. The duration of foraging trips and the total distance travelled, were, on average, 5.1 days and 723 km during the incubation period and 3.0 days and 578 km during chick rearing. The diet of WFSP was dominated by fish and cephalopods (crustacean prey were not detected), with Myctophidae (FO = 71%) representing the main fish family. WFSP often consume mesopelagic fish, in line with their preference for deep oceanic waters and with a small difference in at sea behavior (i.e., travel speed) between the diurnal and nocturnal period. The relatively high concentrations of mercury accumulated in body feathers of WFSP chicks (3.45 ± 1.44 mg kg dry weight; range 1.68–6.01 mg kg) support the idea that WFSP raise their chicks mostly on mesopelagic prey from deep pelagic areas.