PROP J & EERDEN MR VAN (1981) The occurrence of migratory birds in the Lauwerszee area from the embankment in 1969 through 1978. LIMOSA 54 (1): 1-16.

Following embankment of the Lauwerszee in 1969, ten years of succession in vegetation caused marked changes in the occurrence of migratory and wintering bird populations. Detailed studies of plant colonization on 'the sand flats formed the base for the description of the various relations between birds and their food.
      Figure 1 maps the area where over 5000 ha of the former sea bottom could develop in a natural way. The saline environment, still maintained in large parts of the area after ten years gave rise to the colonization of a vegetation mainly consisting of pioneers as Glasswort Salicornia spp. The large seed crop in autumn provides an attractive food for waterfowl as Wigeon Anas penelope and Barnacle Goose Branta leucopsis or a filter feeder as Teal Anas crecca. For each of these three species the maximum population present may amount to at least 50 000 birds (see table 5). Figure 2 gives the total number of bird days for each winter season (August-April) spent in the Lauwerszee area by geese, dabbling ducks and diving ducks. Dabbling ducks arrive early in autumn and are able to exploit the crop maximally compared to the later arriving geese. Also in midwinter and spring mainly Wigeon starts feeding recently on the newly' arrived grasses Puccinellia spp. and Agrostis stolonifera. Both facts may explain the increase in usage of the area by these ducks. More in detail figure 6 gives the changes in yearly production of Salicornia and the yearly food consumption by Wigeon and Barnacle Goose in the area. After a peak production and consumption in the fourth year, production slowly falls but total consumption does so to a much lesser extent due to the increased consumption of perennial grasses. Even within one season there is a marked change in diet of the Wigeon (figure 10). The birds seem to prefer a seed rich diet in autumn and change to grasses in late fall when the ground water level has come to the surface.
      A typical grass feeder as Greylag Goose Anser anser is increasing as it is in Bewick's Swan Cygnus columbianus bewickii which is increasingly profiting by the expansion of the submerse vegetation Potamogeton spp. (figure 4). Otherwise Mallard Anas platyrhynchos and Pintail Anas acuta which forage in late summer on spilled cereals after harvest do not show any overall trend (figure 15). For the relationship between agricultural harvest and the presence of Mallards see figure 13. Compared to the herbivores, the carnivore bird populations that occur in the area are still quite small. Partially this is due to the small amount of foraging area (about 150 ha of fresh water mud) but may be also due to the slow colonization by fresh water molluscs of the deeper parts of the lake. Tufted Duck Aythya fuligula is only present in moderate numbers while no trend is obvious for diving ducks in general (figure 2).
      Several species of waders (table 2) that forage on small bottom dwelling prey species (Chironomidae, Tubificidae) predominantly occur in the summer months when biomass is highest (figure 18). The same holds for Shellduck Tadorna tadorna (figure 5) a pattern strongly diverging from the adjacent coastal populations. Various figures give monthly means, minimum and maximum of the main species of waterfowl that occur in the area. Table 3 lists maximum number of some raptor species according to month while table 4 lists maximum numbers of some granivore songbirds.
      It is concluded that though severe losses occurred after embankment, the Lauwerszee area is still one of the most prominent halting places for migratory birds, mainly waterfowl. Attention has to be paid to threatening features as various planned human activities but also to nature itself by current succession of the vegetation. This progress which definitely leads to a final stage of minor value for waterfowl needs to be studied in detail further on but also nature management plans should be made (Joenje 1978) to optimize the use that the mentioned species of birds can make of the area. Bewick's Swan Cygnus bewickii Taiga Bean Goose Anser fabalis Greylag Goose Anser anser Barnacle Goose Branta leucopsis Common Shelduck Tadorna tadorna Eurasian Wigeon Anas penelope Common Teal Anas crecca Mallard Anas platyrhynchos Northern Pintail Anas acuta Northern Shoveler Anas clypeata = Spatula clypeata Common Pochard Aythya ferina Common Goldeneye Bucephala clangula Eurasian Oystercatcher Haematopus ostralegus Common Ringed Plover Charadrius hiaticula European Golden Plover Pluvialis apricaria Northern Lapwing Vanellus vanellus Little Stint Calidris minuta Temminck's Stint Calidris temminckii Curlew Sandpiper Calidris ferruginea Dunlin Calidris alpina = Calidris alpina schinzii Ruff Philomachus pugnax Black-tailed Godwit Limosa limosa Whimbrel Numenius phaeopus Eurasian Curlew Numenius arquata Green Sandpiper Tringa ochropus Wood Sandpiper Tringa glareola Common Sandpiper Actitis hypoleucos = Tringa hypoleucos Red-necked Phalarope Phalaropus lobatus Eurasian Spoonbill Platalea leucorodia Great Cormorant Phalacrocorax carbo Western Marsh Harrier Circus aeruginosus Hen Harrier Circus cyaneus Common Buzzard Buteo buteo Rough-legged Buzzard Buteo lagopus Common Kestrel Falco tinnunculus Merlin Falco columbarius Peregrine Falcon Falco peregrinus Horned Lark Eremophila alpestris Brambling Fringilla montifringilla European Greenfinch Carduelis chloris Twite Carduelis flavirostris Lapland Longspur Calcarius lapponicus Snow Bunting Plectrophenax nivalis

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