Wolff W.J. (1969) Distribution of non-breeding waders in an estuarine area in relation to the distribution of their food organisms. ARDEA 57 (1-2): 1-28

1. The general distribution of the infauna of the tidal flats of the estuarine area of the rivers Rhine, Meuse and Scheldt in the south-western part of the Netherlands is described. 2. From the occurrence of the infauna it is possible to distinguish (1) a beach district, (2) a marine district, (3) a marine transitional district, (4) a brackish district, (5) a fresh-water transitional district and (6) a fresh-water tidal district. Each of these districts is characterized by a certain combination of typical species. 3. The distribution of eight species of waders shows a correlation with the distribution of their principal food species. For certain other species, the distribution shows correlation with other environmental factors such as exposure to wave-action, nature of the substrate and availability of cover. It is possible to distinguish six groups of species of waders, each with its own characteristic distribution. 4. Group A comprises Purple Sandpiper and Sanderling, which only visit district 1 formed by the exposed shores of the North Sea proper. Their occurrence here is probably correlated with the greater amount of wave-action in this district. 5. Group B comprises Oystercatcher, Knot and Turnstone, only present in district 2, formed by areas with a mean salinity above 13ëCl'. This distribution seems to be correlated, at least for the first two species, with the distribution of their principal food, which is formed by some species of molluscs. 6. Group C comprises Curlew, Bar-tailed Godwit, Dunlin, Grey Plover and Ringed Plover, which are only observed in districts 2, 3 and 4, formed by the areas with a salinity above about 1ë Cl'. This distribution pattern too seems to be correlated with the distribution of food species and especially with the distribution of Nereis diversicolor, Macoma balthica and Carcinus maenas. 7. Group D is formed by the Avocet, which occurs mostly in district 4 consisting of brackish areas with salinities between 1 and 10ë Cl', but under special circumstances (muddy areas) the Avocet also is seen in areas with higher salinities. 8. Group E comprises Snipe, Ruff, Lapwing and Black-tailed Godwit, which are only found in districts 4, 5 and 6, where salinity is below 10ë Cl'. The distribution of the first of these species is correlated with the amount of cover available; for the distribution pattern of the others, no explanation is available. 9. Group F consists of Greenshank, Redshank, Spotted Redshank and Common Sandpiper, which occur in all the districts mentioned. This may be correlated with the fact that their type of food is present in all these districts.