Blijvend slechte reproductie van Kokmeeuwen Larus ridibundus op een binnenlandse broedplaats

Only ten years ago, inland colonies of Black-headed Gulls abounded in The Netherlands. One such colony was situated on Doldersurnmerveld, a heath of 230 ha in western Drenthe. In the 1990s, this site held a colony in 1994 (85 pairs), 1995 (15), 1999 (13) and 2000 (17), but of 1000 pairs in 1983. Nesting success and feeding ecology were studied in the latter three years. It was found that breeding was restricted to years with a high water table; this happened only four times during 1990-2000 (see above, Fig. 1). Due to intensive grazing and removal of the top layer in large parts of the heath, Molinia caerulea and Juncus effusus were largely eradicated in the 1980s, thus reducing the number of nesting sites. Mean nearest nesting distances varied between 21 and 23 m in 1995, 1999 and 2000, showing that nests were quite far apart. In combination with small colony size (actually even smaller because of division into 2-3 sub-colonies), this turned out to be a problem during defensive actions against avian predators as large gulls. Goshawk Accipiter gentilis and Common Buzzard Buteo buteo. Combined attacks against raiding predators were hardly effective when involving only handfuls of birds. Egg-laying in 1995 and 1999 started in late May, in 2000 in early May, median dates being respectively 5-9 June, 3 June and 9 May (Fig. 2). Synchronisation of laying was poor, the laying period covering resp, 13, 8 and 11 days in 1995, 1999 and 2000. Re-laying after egg losses was recorded in 1995, but not at all or rarely in other years; this was probably due to the receding waterline, enabling Red Foxes to enter the fringes of the colony. Clutch size did not differ much between years (Table 1), but egg size did (Table 2). Nesting success was exceedingly poor: a single fledgling out of 71 nesting attempts in three years (Table 3). Condition of the chicks was also poor (Fig. 3), i.e. weighing much less than chicks in a successful colony in the Wadden Sea. Very few chicks reached ages of >7 days old. In most cases foraging flights of breeding Black-headed Gulls exceeded 2 km (Fig. 4), with several flights probably >10 km long. The immediate surrounding of the colony was unsuitable for foraging, being covered with woodland, heaths, rough pasture and nature reserves where mowing was delayed (till after 15 June, in some years till after late July). Farmland had become increasingly unsuitable as feeding area since the 1970s, as a result of lowered water tables, reseeding with highly productie single-species grasses, changes in manuring systems and conversion of farmland into green maize and bulb-growing fields. It is thought that the demise of inland colonies of Black-headed Gulls is mainly food-related, either by changes in crop choice and farming practices and/or by declining availability of profitably exploitable food (lowered water table resulting in dry grasslands).