Broedende Grote Sterns Sterna sandvicensis op Hompelvoet en Markenje, 1989 – 1992

Since 1986 there has been a slow but steady decline in the numbers of breeding Sandwich Terns at the Hompelvoet/Markenje colony (lake Grevelingen; figure 1), the second largest in The Netherlands. Since 1989 the number of breeding pairs decreased with 15% per annum to 1575 pairs in 1992 (table 2). During 1979-1988 an average of 3800 pairs bred. Clutch size and breeding success were comparatively low during 1990-1992. It is assumed that most birds moved to breeding locations near Zeebrugge (Belgium) and Hooge Plaaten (Westerschelde) which became suitable by this time. Furthermore, predation and disturbance (mainly by Stoat, rats, gulls and Marsh Harriers) were important factors causing the departure of terns. A possibly more structural decline in food availability is worth further research.