Reproduktieve treurnis in een kolonie Kokmeeuwen Larus ridibundus

After several years of absence because of drought. Black-headed Gulls recolonised Doldersummerveld (a wet heath in western Drenthe) in 1994, but fox predation prevented successful breeding. The site was again occupied in 1995. Fifteen pairs produced 34 clutches, with a minimum of 66 eggs. Mean clutch size in completed clutches was 2.1, i.e. 4x 1, 3x 2 and 5x 3 eggs/clutch. Only a single young reached fledgling status (Fig. 1). Although the water level remained high enough throughout the summer to discourage foxes from invading the colony, egg predation was still an important cause of failure, this time by a Herring Gull, three Lesser Black-backed Gulls and several Carrion Crows. Successful predation was possible because the already small colony was divided in three sub-colonies, each some 100-160 m apart. Therefore, potential predators were only attacked by birds from one sub-colony at the lime, reducing the effectiveness of cooperative attacks. Moreover, feeding conditions were very poor, following a cold and wet June and an extremely dry July. For food, breeding birds had to commute between colony and grassland over distances of 3 km or more; this situation was aggravated by drought conditions in July (reduced availability of earthworms). All hatched young had low body masses relative to their age (Fig. 2) and several emaciated young were found. Except for one young, all hatched young eventually died from starvation. Whereas mean egg size (51.3x36.5 mm, n=26) did not differ from the western European mean (cf. Holz & Starke 1991) and all breeding birds were in adult plumage, it is thought that drought, resulting in prolonged absence of Black-headed Gulls in the colony, an increase in egg predation, depleted food supply and starvation of young, was the main cause of failure in this colony. Throughout the province of Drenthe, the water table has been lowered considerably during the last few decades, and this may -by reducing food availability- have triggered the overall decline of the Black-headed Gull as a breeding bird.