Stormmeeuwen Larus canus op zee in het broedseizoen

At the seawatching site Hondsbossche Zeewering (fig. 1) along the coast of the province of Noord-Holland (Netherlands) summer seawatches always show remarkable numbers of adult Common Gulls, reaching peak numbers in the last two weeks of June and the first two weeks of July, dropping immediately to virtually nothing until October (fig. 2). As shown by fig. 1 there are some colonies of this species in the immediate vicinity of the seawatching site (in the dunes of the village of Schoorl), amounting to a total of 3000 – 5000 breeding pairs. Undoubtedly foraging birds from this colony are to be held responsible for the peak numbers observed at sea. Adult Common Gulls arrive at the colony by March and start laying in the second week of May (Arbouw 1985). It is not, however, until the second half of June that significant numbers fly out to the sea. As widely known (e.g. Vernon 1971, Arbouw & Swennen 1985), Common Gulls are more inclined to inland foraging than most gulls. They keep doing exclusively this until the hatching of the chicks. Then either the increased need of food or the decreased availability of worms (their favorite prey in the meadows, Arbouw & Swennen 1985, PLatteeuw 1986) by the drying out of the soil or a combination of both urges the birds to go out to the sea as well. There they probably take advantage of aggregations of small fish species like young Herring, Sprat and Sand-Eel being forced close to the surface by hunting Mackerel. They do forage near fishing vessels and at the beach as well. At the beach they pick up the edible waste left behind by thoughtless tourists. From observations in the nearby Schoorl colony we learned that indeed, although the first feedings of the day still consisted mainly of worms (early in the morning they still go out to the meadows, just to reach peak numbers at sea around mid-day, figs. 3 & 4), while later on fish and offal were fed. In summer, when worms are getting harder to find (at least at mid-day) and the chicks require a lot of food, Common Gulls seem to go out first to the meadows as usual to forage on worms. Later on, however, as the meadows go dry, they switch to the coast, where they presumably soar above the dunes and keep a keen eye on whatever happens at sea or on the beach. If a flock of fishing Herring Gulls and Lesser Black-backed Gulls occurs or a fishing vessel appears they fly out offshore to join the party. From the seawtching site more birds are observed flying from the colony (that is northwards) than towards the colony (southwards). This may be due to the fact that flying out they keep close to the sea surface searching for food, while on the way back they are fully satisfied and do no longer have the need to search. They probably then fly higher and more directly towards the colony and thus don’t pass the binocular field of view of us seawatchers.