Non‐Breeding Distribution and Activity Patterns of Antarctic Storm‐Petrels Inferred From Geolocator‐Immersion Loggers and Stable Isotopes

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ABSTRACT Outside the breeding season, small pelagic seabirds are particularly challenging to study due to their large geographic ranges and high mobility. Consequently, their non‐breeding distribution and activity patterns are often poorly known. However, the non‐breeding period is of critical importance, encompassing both migration and flight feather moult. We combined miniaturised geolocator‐immersion loggers (GLS) with carbon and nitrogen bulk and compound‐specific stable isotope analyses of rectrix feathers. Using this approach, we investigated the annual phenology, migratory routes, moult locations, non‐breeding activity patterns and distribution of two Antarctic storm‐petrel species: Wilson's Storm‐petrel Oceanites oceanicus and Black‐bellied Storm‐petrel Fregetta tropica . Stable isotope data revealed a narrower isotopic niche and less intraspecific variation in trophic positions for Black‐bellied Storm‐petrels ( n = 15) compared to Wilson's Storm‐petrels ( n = 15). GLS data suggested the non‐breeding season of Wilson's Storm‐petrels to last from around mid‐April to end of November. All individuals ( n = 7) migrated transequatorial in a clockwise pattern. Three individuals initially stayed at a non‐breeding site in the South Atlantic Ocean. The main non‐breeding sites of all individuals were in the North Atlantic Ocean, along the eastern coast of Canada and the United States, where flight feather moult took place from July to September, inferred from a period of reduced flight activity (measured by time on water). The foraging activity of Wilson's Storm‐petrels was higher at night than during the day. This was not influenced by lunar phase but is likely linked to prey behaviour. The single successfully tracked Black‐bellied Storm‐petrel also crossed the equator but spent the non‐breeding period in the Indian Ocean, mainly in the Arabian Sea. This indicates spatial segregation from Wilson's Storm‐petrels. We present the first ever GLS tracking data of Wilson's and Black‐bellied Storm‐petrels, improving our understanding of their non‐breeding behaviour. Future research including more individuals alongside environmental variables will be necessary to examine the drivers of the observed inter‐ and intraspecific non‐breeding spatial and trophic segregation.

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