NIENHUIS J (2022) Night-time foraging for earthworms by Black-headed Gulls Larus ridibundus. LIMOSA 95 (1): 41-45.

The Black-headed Gull is a diurnal species with a preference for earthworms. They often look for this protein-rich prey during or shortly after rainfall. On two occasions, gulls were observed feeding on earthworms in the city of Groningen during the night. This made me wonder how common this behaviour was. The first study site, Sontplein, is a site where Black-headed Gulls forage on fast-food leftovers. Here they are known to roost of roofs. The second study site, Oostersluis, is a more traditional night-roost site near water. During frequent night-time visits to both locations in the winter of 2016/17, earthworm-seeking Black-headed Gulls were counted. In all cases, the gulls walked on lawns lit by streetlamps along access roads. In 40% of the nights, this worm-seeking behaviour was observed on at least one of the locations. The number of birds showing this behaviour was on average 38 at Oostersluis and 75 at Sontplein. The maximum number of night-time feeding gulls observed during one night was 300. From resightings of ringed birds and ring densities it was estimated that a total of about 650 birds would be engaged in night-time foraging for earthworms during December and January.
Night-time foraging occurred more often before the 1st of January (37% of nights) than afterwards (17%). Also, it varied during the night. Night-time foraging was not observed prior to 8 pm. Subsequently it peaked between 8 and 10 pm (46% of nights), to gradually decrease until 4 am, after which no worm-seeking behaviour was observed anymore. Higher temperature and air humidity had a positive effect on the number of night-time foraging gulls, as did rainfall - when time progressed after rainfall, the number of worm-seeking gulls decreased.
Black-headed Gulls are not adapted to a nocturnal lifestyle. They normally do not feed at night. The night-time foraging reported in this study occurred under streetlight, suggesting that Black-headed Gulls have difficulties finding prey in the dark. It would be interesting to determine food intake rates during the day and at night, also in relation to weather conditions. This can provide insight into the decision to forage during the day or at night.

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