ENS B J, B AARTS, K OOSTERBEEK, M ROODBERGEN, H SIERDSEMA, R SLATERUS & W TEUNISSEN (2009) Studies into the causes of the dramatic decline of the Oystercatcher Haematopus ostralegus in the Netherlands. LIMOSA 82 (2): 83-92.

The number of Oystercatchers in The Netherlands is declining rapidly (Fig. 1, 2). This is cause for alarm, because the Netherlands is home to a large part of the wintering and breeding Oystercatchers in Europe and because Oystercatchers are extremely long-lived, so rapid declines only occur when conditions have drastically deteriorated. To bring this decline to the attention of the general public and to study its causes, BirdLife Netherlands and SOVON Dutch Centre for Field Ornithology proclaimed 2008 the 'Year of the Oystercatcher' in The Netherlands. The decline in the Oystercatcher population became noticeable around 1990 and was discussed at a special symposium in 2003 (Ens et al. 2003). Many of the questions posed there are still open today. As part of the 'Year of the Oystercatcher', we tried to assemble data collected by observers that has not been stored in the SOVON database, so until now unavailable for analysis. This enterprise was particularly successful for counts of clubs and night roosts. We also stimulated volunteers to map breeding Oystercatchers to produce a density map of the Netherlands. We used data from the national breeding bird monitoring scheme to produce a statistical model of breeding density as a function of landscape variables, which we then extrapolated to all parts of The Netherlands (Fig. 3). We asked observers to pay special attention to birds breeding on flat roofs in and around urban areas. Most Oystercatchers were found breeding on roofs with pebbles. Although mortality may have increased, the available evidence suggests that the prime demographic cause of decline is a decrease in reproductive success. Hence our efforts to stimulate volunteers to score reproductive success of Oystercatchers (Fig. 4) in the same areas where breeding pairs were mapped. The data remain to be analyzed in detail, but on average reproductive success in 2008 was too low to sustain the population. We hope to find out under what conditions Oystercatchers do successfully raise young. Because Oystercatchers are long-lived and because conditions are quite variable between years, a one-year effort is unlikely to provide all the answers. Hence the need for long-term population studies of individually marked birds (Fig. 5). We developed a website where observers can input their observations of marked individuals and get immediate access to all previous observations of that particular individual: www.wadertrack.nl. We also stimulated groups of volunteers to set up new long-term population studies (Fig. 6). We are confident that this will help us understand in the long run why Oystercatcher populations are declining so rapidly.

[pdf only for members]