VOSLAMBER B (1992) Egrets Egretta sp. in Oostvaardersplassen in 1991. LIMOSA 65 (3): 89-92.

Great White - and Little Egrets have been recorded annually since 1975 in the extensive marsh area Oostvaardersplassen, located in one of the recently reclaimed polders in Lake Ilssel, The Netherlands. One pair of Great White Egrets bred successfully here in 1978. Since then the species probably bred in most years (1-2 pairs), but without success; this may have been due to local food shortage. Little Egrets successfully bred in Oostvaardersplassen in 1979, but not since then; in 1989 and 1990 this species was not recorded in the marsh. In an attempt to study and improve feeding conditions for egrets and other long-legged waders, an experimental area with ditches and pools (De Waterlanden, 11 ha) was constructed just outside Oostvaardersplassen in 1986. Water level in the experimental area was manipulated. The area proved attractive for foraging egrets, Spoonbill and Grey Heron (1987-90). However, the breeding situation of Great White Egrets did not improve. Manipulation of water level in Oostvaardersplassen included letting dry up a western section (about 1000 ha) in 1987. Following copious rainfall in winter 1990/91, this section again had water in spring 1991. Unlike earlier, the shallow water was now transparant, and the abundance of zooplankton (Daphnia) and Chironomid larvae and young fish (sticklebacks) feeding on these resulted in attractive feeding conditions for visually foraging waders. Unprecedented numbers of egrets were recorded in the marsh (mainly western section) in summer 1991: up to at least eight Great White- and five Little Egrets. One pair of Great Whites successfully raised three young. Several other Ardeids (Bittern, Black-crowned Nightheron, Grey- and Purple Heron) and Spoonbills were relatively common in the marsh during spring-summer 1991. Good foraging conditions for egrets and other long-legged waders are expected again in 1992. Little Egret Egretta garzetta Western Great Egret Ardea alba = Egretta alba

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