BIE S, DE & ZIJLSTRA, M (1985) Avoeets Recurvirostra avocetta in the Oostvaardersplassen: freshwater marshes for safe breeding?. LIMOSA 58 (2): 41-48.

Pied Avocet Recurvirostra avosetta Avocets Recurvirostra avosetta invaded the IJsselmeerpolders quickly after the polders were reclaimed. As a result of lack of tidal movements and their freshwater nature these polders differ from the traditional Avocet habitat. As 75% of the Dutch Avocet population breed in threatened areas, reclaimed polders gain importance which warrants a study of the biology of the Avocet in these areas. Since 1968, large numbers of Avocets (fig. 2) visit the Oostvaardersplassen (Z-Flevoland, fig. 1). Numbers fluctuate with the water level (fig. 3). However, wind and water movements, responsible for silt deposition and erosion, change the foraging possibilities in the Oostvaardersplassen continuously. Consequently the maximum number of Avocets may differ from year to year. The numbers of breeding Avocets in the Oostvaardersplassen during 1968-82 are given (fig. 4); largest numbers are found at a water level of -3.70 m NAP. Breeding occurs in or near the feeding grounds at sites situated 5-10 cm above the actual water level. No relationship with the vegetation type was found. Egg laying is synchronized in all colonies although it appears to be stronger in colonies close to the water edge. During the period 1973-75, the average clutch size was 3.79 (ct. tab. 2). Average hatching success differred between colonies inside and outside the marsh. Inside the marsh, egg losses were considerable, mainly due to a sudden rise of the water table; outside the marsh, predation was the main factor. Comparing the biology of Avocetsin the Oostvaardersplassen with that in a coastal habitat, it appears that in both the water level is a crucial factor. It is argued that Avocets starting to breed in areas like the Oostvaardersplassen have equal of even worser prospects than in tidal areas due to the long term unpredictability of the water level and the lack of alternative areas when the water level is too low (no feeding grounds) or to high (no breeding and feeding grounds).

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