OUWENEEL GL (1973) The avifauna in the HoIlandsch Diep-Haringv1iet area during the first two years after the closure. LIMOSA 46 (3): 166-191.

As a part of the Delta-works on 2 November 1970 the Hollandsch Diep-Haringvliet was closed from the North Sea. Before the closure the river was under the influence of the tides. At low tide a lot of flats? emerged along the shore as well as in the river. Important flats are the Slijkplaat, the Ventjagersplaat and the Sasseplaat. 2. After the closure the tidal difference was reduced to about 15 centimetres,. The Department of Waterways regulates the watedevel by big sluices. After the, closure about 70% of the flats disappeared permanentely under water; now only parts of the flats are dry irregularly. Results of the closure are decrease of the strength of current and a decrease of the salinity. 3. Before the closure of the river every winter the grasslands and reedlands outside the dikes were flooded irregularly. This was, finished after the closure. The diminishing of the salinity and the drying up of the grounds outside the dikes resulted in the introduction of many new plants. The reedlands changed in character and the formerly vast stretches of rush were reduced. On the flats which now are permanently dry a varied vegetation developed quickly. 4. The principal haunts are mentioned. During the two years after the closure the author visited these haunts regularly. The Hoogezandsche Gorzen, the Ventjagersplaat and the Korendijkse, Slikken were visited at least once every fortnight. These three haunts are rather good examples of the principal types of haunts in the area. 5. The area is a principal haunt for waterfowl and waders. At the end of august 1966 in this area, including the Biesbosch, were counted 39.000 MaUards, 12.000 Teal, 2120 Snipes, 1300 Curlews and 7700 Avocets. Moreover the area is a very important haunt for several species of Geese. In autumn and spring about 10.000 Grey Lag Geese can stay in the area, some thousands are wintering and since a few years this species is spending its wingmoult on the Ventjagersp1aat. In winter White-fronted Ge:ese, Bean Geese and Barnacle Geese are numerous. 6. The status of the breeding-birds during the two years after the closure is described. The Great Crested Grebe first bred in 1972 with only one pair. It was generally expected that this bird would settle more e<.~i1y after the elimination of the tides. The colony of Grey Herons on the Hoogezandsche Gorzen increased from 9 nests in 1970 till 25-30 nests in 1972. The breeding-stock of the several Anatidae up till now, does not seem to have changed much after the closure. The Gadwall increased but this is noted on several places in the Netherlands. In 1971 and 1972 a pair of Grey Lag Geese attempted to breed on the Hoogezandsche Gorzen. The breeding-stock of the Marsh Harrier in the area varies between 3 and 6 pairs. A number of non-breeding Marsh Harriers was spending the summer of 1972 in the area. In the first year after the closure 50---60 pairs of Kentish Plovers, about 10 pairs of Ringed Plovers and some pairs of Oystercatchers and Avocets settled on a bank that was left permanentely dry in the neighbourhood of the Ventjagersplaat. In 1972 the quick development of the vegetation made this bank less attractive for breeding waders. Probably the increase of the breeding-stock of the Short-eared Owl correlates with the increase of the small rodents on the grounds outside the dikes. In 1970 only one pair bred in the area; in 1972 at least 5. The Marsh Warbler increased quickly in numbers in the last few years. Without doubt the expansion of the rough varied vegetation in the reedlands correlates with this increase. 7. Changes in the numbers of the non-breeding birds are described. Concerning the Anatidae: Teal decreased to about 15-20% of the pre-closure numbers; the Pintail disappeared almost completely. An increase in the numbers of diving-ducks was observed. The summering non-breeding population of the Shelduck did not change. On the whole the numbers of the wintering geese did not show remarkable changes In 1970-1971 the concentrations reached their normal size. In 1971-1972 less Bean Geese, Whitefronts and Barnacle Geese wintered in the area, but certainly the mild winter and the low number of White-fronted Geese and Barnacle Geese in first year's plumage played a part. In both years the number of Grey Lag Geese during the springmigration was low in comparison with the pre-closure years. The moultingconcentration of Grey Lags on the Ventjagersplaat decreased from about 1100 in 1969 to 110 birds in the summer of 1972. Most of the wader-species showed a striking decrease in numbers after the closure. The species which prefer a marine biotope, disappeared almost completely. Only the Ventjagersplaat, south of which a marine-tidal area ends in extensive mudflats, kept a rather important concentration of waders. On the whole, however, the Curlew lost about 85% of its pre-closure number and the' same decline showed the Grey Plover, the Dunlin, and the Bartailed Godwit. The Ventjagersplaat is an important area for both Redshank and Ruff. On 11-9-1971 3000 Ruffs were counted on this spot. A serious decline in the number of Avocets has been determined. A number of 7700 specimens were counted in august 1966. In the following years this number decreased to about 1250 birds in august 1971. In july 1972 2500 Avocets were counted. Great Crested Grebe as well as Cormorant increased quickly as non-breeding visitors. In the autumn of 1972 300' Cormorants have been counted in the whole area. Formerly this area was important as a concentration-place for Spoonbills after the breeding-season. The decrease in numbers already began some years before the closure but did not decline further in 1972. ##### Great Crested Grebe Podiceps cristatus Great Cormorant Phalacrocorax carbo Grey Heron Ardea cinerea Eurasian Spoonbill Platalea leucorodia Mallard Anas platyrhynchos Common Teal Anas crecca Garganey Anas querquedula Gadwall Anas strepera Eurasian Wigeon Anas penelope Northern Shoveler Anas clypeata = Spatula clypeata Common Shelduck Tadorna tadorna Greylag Goose Anser anser Mute Swan Cygnus olor Eurasian Oystercatcher Haematopus ostralegus Northern Lapwing Vanellus vanellus Grey Plover Pluvialis squatarola European Golden Plover Pluvialis apricaria Common Ringed Plover Charadrius hiaticula Kentish Plover Charadrius alexandrinus Ruddy Turnstone Arenaria interpres Common Snipe Gallinago gallinago Eurasian Curlew Numenius arquata Whimbrel Numenius phaeopus Bar-tailed Godwit Limosa lapponica Wood Sandpiper Tringa glareola Common Sandpiper Actitis hypoleucos = Tringa hypoleucos Common Redshank Tringa totanus Common Greenshank Tringa nebularia Spotted Redshank Tringa erythropus Red Knot Calidris canutus Little Stint Calidris minuta Dunlin Calidris alpina = Calidris alpina schinzii Curlew Sandpiper Calidris ferruginea Ruff Philomachus pugnax Pied Avocet Recurvirostra avosetta Black Tern Chlidonias niger Common Tern Sterna hirundo Little Tern Sternula albifrons Sandwich Tern Thalasseus sandvicensis = Sterna sandvicensis

[free pdf] [dutch summary]