Under the cover of darkness: Foraging behaviour increases on cloudy, new moon nights in a pelagic seabird

While many visual predators feed exclusively during the day, others exploit foraging opportunities at night, although the efficiency with which they do so is strongly influenced by ambient light levels. We investigated the influence of variation in nocturnal light on the foraging behaviour of a small pelagic seabird, the European Storm‐petrel Hydrobates pelagicus . Nocturnal foraging, identified using GPS‐accelerometry data and a Hidden Markov Model, was concentrated in two areas: shallow coastal waters near the colony and deeper offshore waters near the shelf edge. Generalized additive models revealed that, in offshore areas, European Storm‐petrels forage under the darkest nocturnal conditions, contrasting with the increased activity under moonlit conditions reported for many other nocturnally foraging seabirds. This was strongly influenced by cloud cover and suggests that European Storm‐petrels may be exploiting vertically migrating prey that are more accessible under low ambient light. While olfaction probably plays an important role in detecting prey patches at night, bioluminescence may aid visual detection of individual prey items. In contrast, nearshore foraging was unrelated to nocturnal light and was instead associated with local productivity indicators. If the trend of declining cloud cover in the North Atlantic continues, reducing the frequency of dark nights, European Storm‐petrels may alter their nocturnal foraging strategies, forcing them to rely primarily on coastal regions. Our results support growing evidence for the idea that climate‐driven and anthropogenic changes to sensory environments are likely to alter the foraging ecology of nocturnal species.