Evidence for nocturnal inter-tidal foraging by European Storm-petrels Hydrobates pelagicus during migration

European Storm-petrels Hydrobates pelagicus have previously been assumed to be exclusively pelagic foragers during migration. However, in this paper we report evidence that migrating Storm-petrels also forage at night along beaches. We highlight the repeated occurrence of the inter-tidal crustaceans Eurydice naylori & E. affinis (Isopoda: Cirolanidae) in the regurgitated crop contents of European Storm-petrels captured for ringing during their northwards migration past SW Portugal. The combination of the fresh condition of these crustaceans, their habitat and limited intertidal distribution and their nocturnal pattern of activity, together indicate that the Stormpetrels which had eaten them had been foraging by night along the inter-tidal zone of sandy beaches. We also found subtidal Eurydice species in the regurgitated samples, including the offshore species E. inermis and E. truncata that are nocturnal vertical migrants to the sea surface, providing further clues as to the location and timing of Storm-petrel foraging. We highlight the insights into the foraging behaviour of migrating Storm-petrels that can be obtained from the study of their gut contents and the behaviour and ecology of their prey.