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
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.
[pdf only for members] [dutch summary]