Trekbewegingen van Kokmeeuwen Larus ridibundus langs de Noordzeekust: oorzaken en achtergronden

Based on regular seawatches during 1980-84 in three Dutch regions. Wadden area (WD), Noord-Holland (NH) and Zuid-Holland (ZH), a description is given of the seasonal migration pattern of the Black-headed Gull, The seasonal pattern appeared to be reasonably constant thoughout the years, with two spring peaks (Tebruary/March and April) and two major autumn peaks (July and October). Hard weather movements occurred irregularly during frost and snow in winter. The first spring peak consisted of adult birds returning to Dutch breeding colonies and probably of Danish and German birds as well (cf. MacKinnon & Coulson 1987), The second one consisted of adult birds in the first half of April, probably of Baltic origin, and of 2nd cy birds in the rest of the month. Spring migration was strongest in the south , whilst in the north many birds passed by the IJsselmeer, probably attracted by spawning Smelt Osmerus eperlanus. In autumn the first half of July showed mainly adult birds moving south. Possibly these birds were 2nd cy individuals having completed their post-nuptial moult and returning early to the wintering areas (cf. MacKinnon & Coulson 1987), The second half of July showed predominantly 1st cy birds, probably of Dutch origin. This predominance of immatures lasted till well into August. Inland numbers wre relatively low at this time of the year, though a migration peak was observed here as well demonstrating the exodus (Lensink & Kwak 1985), A drying out of the pastures can be held responsible for this first autumn peak. In Wadden Sea and IJsselmeer however food resources seem ideal in August and thus attract peak numbers of Black-headed Gulls, In October these food resources get less abundant, which forces many birds to move southwards. This migration peak is virtually only detectable along the coast. Inland numbers have grown in the meantime, caused by influx from Finland and the Baltic. Though many of these merely pass by, huge numbers winter at inland pasture areas, where the food situation has become much more favorable. Only frost and snow can now still force them out. Virtually all October migrants are adults, as well as all birds involved in hard weather movements. This strongly suggests that immature birds try to avoid competition for food with the more experienced adults by migrating further afield.