DIJK K VAN & R OOSTERHUIS (2010) Origin and trends in numbers and breeding success of Black-headed Gulls Larus ridibundus breeding on griend (Wadden Sea). LIMOSA 83 (1): 21-35.

The island of Griend in the Dutch Wadden Sea (53°15'N, 05°15'E) harbours a large breeding colony of Sandwich Tern Sterna sandvicensis, and for a long period the breeding Black-headed Gulls were considered to have harmful effects on nesting terns. Therefore, every year all gull eggs were removed up to 1968. After this year both species showed a steep increase (Fig. 6). Griend currently holds around 30,000 breeding pairs of Black-headed Gull (Fig. 1), 25% of the Dutch population and 61% of the population in the Dutch part of the Wadden Sea. Breeding success on Griend averaged 1.0 fledged young per breeding pair in 1994-2008 (Fig. 2), indicating that Griend is a favourable breeding area. It is argued that data on breeding success are only reliable when measured in fenced enclosures.
      We analysed 170 recoveries of 109 ringed Black-headed Gulls breeding on Griend in 1999-2001. The recoveries revealed several examples of natal dispersal and breeding dispersal towards Griend, mostly from other colonies in the Wadden Sea (Fig. 5), but also by gulls born in Belgium, France and the UK, indicating that natal dispersal can occur over long distances. Since the eighties Red Foxes Vulpes vulpes recolonized areas along the mainland coast of the Wadden Sea, which caused major changes in the distribution of Black-headed Gulls (Fig. 7). Recoveries show that natal and breeding dispersal from colonies along the mainland coast towards Griend have occurred, and thus the desertion of the mainland colonies will partly have caused the huge numbers currently breeding on Griend. A considerable number of ringed subadults seen on Griend during the breeding season originate from countries around the Baltic Sea (Table 1), but no ringed adults from the Baltic population were seen during the breeding season. This finding agrees with a lack of confirmed breeding records of birds from the Baltic population in the northern part of The Netherlands.
      We discuss various biases that are present in the ring recoveries. The recovery pattern of foreign-ringed birds (Fig. 3) is strongly skewed towards activities of British ringers, and the age pattern of the breeding birds (Fig. 4) is skewed towards our activities on Griend as well. Nevertheless, the recoveries have significantly improved our insight in trends and dispersal of Black-headed Gulls on Griend and in the Wadden Sea.

[pdf only for members] [dutch summary]