Aflezen van metalen ringen bij Kokmeeuwen in Groningen en het Gooi

This paper presents some data on the origin of Blackheaded Gulls Larus ridibundus (BHG) in two Dutch urban areas during the non-breeding season. All data were gathered by frequently reading metal rings with binoculars and a telescope. The study sites are situated in the northern part of the Netherlands (Groningen, 53°13’N/ 06°34’E) and in the central part of the Netherlands (Met Gooi, 52°13’N/05°11’E). In Groningen the study was carried out between 1977-1994 and in Met Gooi the study was carried out between 1984-1994. In the last part of this paper, some information on prenuptial head moult will be presented as well. In total, more than 2300 different BHG-rings from 18 European countries were identified, 985 in Groningen and 1361 in Met Gooi (table 1). The number of different BHGrings identified in one season (between 1 July and 30 June) amounted to a maximum of 433 in Het Gooi (1986/1987) and 227 in Groningen (1989/1990). Data on the maximum number of different BHG-rings from different countries identified in one season are presented in table 2. BHG from northern countries were mainly ringed as a chick. The large number of Dutch BHG-rings observed in Het Gooi is caused by a large-scale ringing program in 1968-1985. Table 3 presents a comparison of the annual average numbers and percentages of foreign BHGrings identified in Groningen and Het Gooi, based on the seven successive seasons (1986/1987 – 1992/1993) when both sites were extensively checked on metal BHG-rings. In this seven years, only four ringed BHGs (two ringed in Het Gooi and one in England and in Sweden) were seen both in Groningen and in Het Gooi. Most foreign BHGs were ringed in the three Baltic countries, 45% in Het Gooi and 47% in Groningen. Site-fidelity is an obvious phenomena of urban BHGs. Table 4 presents data on the return rate of ringed BHGs in Groningen from seven consecutive seasons. N means the number of different BHG-rings identified that season, T means the number of BHG-rings of that year identified one or more seasons later in Groningen. On average, 47% of all ringed BHGs returned one or more seasons later to Groningen. Ringed BHGs which were seen at least two seasons in Groningen have a chance of 70% to return for a third season. Furtheron, most BHGs are very faithful to their own park or pond. The extreme is an Estonian bird wintering twelve successive winters in the Noorderplantsoen in Groningen. The oldest BHGs were seen in Het Gooi: one of 26 years and another one of at least 25 years. The severe winter of 1986/1987 caused a temporary influx of BHGs normally wintering in Denmark or in northern Germany. High numbers of Danish and German rings were seen in Groningen and Het Gooi. Table 5 presents migration data of ten colour-ringed BHGs from Copenhagen to the Netherlands in 1986/1987. In the second column the last observation in Copenhagen is mentioned, in the third column the first observation in Groningen/Het Gooi is mentioned. Almost all of these BHGs returned to Copenhagen, but none of them was ever seen again in the Netherlands. In non-breeding plumage, adult BHGs show several variations in the winter head pattern (figure 2). Individuals can be recognized year after year by their individual differences in winter head pattern. Also, the prenuptial moult of the head differs individually. Figure 3 presents six different moult patterns of individual BHGs. Again, individuals show the same moult pattern year after year.