WHAT DETERMINES THE DENSITIES OF FEEDING BIRDS ON TIDAL FLATS - A CASE-STUDY ON DUNLIN, CALIDRIS-ALPINA, IN THE WADDEN SEA

The tidal and seasonal pattern of habitat utilization by dunlin was st udied in a tidal-flat area in a shallow bay called Konigshafen, island of Sylt, by counting the number of dunlins on marked plots at 10-min intervals over whole tidal cycles. Sediment type, tidal elevation, and seasonal changes in food choice were found to influence the feeding d ensities of dunlin irrespective of total numbers present in the area. Densities of dunlin were generally highest on low muddy substrates. A preference for high sandy substrates was found in late summer. The tid al pattern of habitat utilization differed between areas. In preferred areas dunlin stayed during the whole emersion period. Other areas wer e only used by following the moving tide line. Seasonal changes in hab itat utilization were apparently triggered by changes in food choice. In spring dunlin fed almost exclusively on polychaetes. In late summer a relatively high proportion of shrimps, Crangon crangon, was found i n the diet of dunlins. The preference for shrimps may force the birds to stay away from the tide line, where the shrimps can escape into dee per waters. It is recommended that studies on habitat utilization on t idal flats should consider the tidal behaviour of the target species, as single low-tide counts may not give valid results.