Zwin Natuur Park – Belgium

Category: MBP newsletter content

Published: April 23rd, 2024

Large number of wintering waterbirds at the Zwin nature reserve
Many waterbirds were attracted to the Zwin nature reserve during the winter of 2023-2024. The combined maximum totals of geese, ducks and waders that were counted in the period October – March exceeded 20.000 birds. Various species peaked at different times during this period, so not all these birds were present at the same time. In the peak period of December – January 10.000-15.000 birds were present at the same time.
Up to 6.000 Greater White-fronted Geese (Anser albifrons), up to 4.000 Barnacle Geese (Branta leucopsis) and up to 1.500 Greylag Geese (Anser anser) used the reserve mostly as a night roost. Numbers varied from day to day, but on days when large numbers came in it was quite a spectacle to see flock after flock arriving at dusk, calling loudly. For foraging these geese mostly use the farmland surrounding the reserve, although smaller numbers (several hundreds) also seek food in the reserve.
More than 4.000 ducks wintered in the Zwin. A considerable number of these where Common Shelducks (Tadorna tadorna), foraging in the mudflats of the marine part of the reserve and peaking at 456 birds in November 2023. Other species that were numerous were dabbling duck species, with Mallard (Anas platyrhynchos) (maximum 1.344 birds), Common Teal (Anas crecca) (maximum 1.203 birds) and Eurasian Wigeon (Mareca penelope) (maximum 906 birds) being the most numerous. Smaller numbers of other species were also present, notaby Northern Shoveler (Spatula clypeata) (max. 221), Northern Pintail (Anas acuta) (max. 163), Gadwall (Mareca strepera) (max. 143), Tufted Duck (Aythya fuligula) (max. 54) and small numbers of a few other species.
The mudflats of the Zwin reserve are very attractive for waders, with more than 5.000 birds of various species using the area as a wintering site. The most numerous species were Eurasian Oystercatcher (Haematopus ostralegus) (max. 401 birds), Pied Avocet (Recurvirostra avosetta) (max. 219), Grey Plover (Pluvialis squatarola) (max. 191), Northern Lapwing (Vanellus vanellus) (max. ca. 2.000), Eurasian Curlew (Numenius arquata) (max. 568), Dunlin (Calidris alpina) (max. 1.512) and Common redshank (Tringa totanus) (max. 434). Especially the number of Dunlin that wintered in the reserve in the winter 2023-2024 was impressive. Only a decade ago, the habitat for this species had decreased so much that the wintering population did not even exceed 100 birds in some winters.
The number of waterbirds using the reserve has considerably increased in the past five years. Especially wader numbers have increased strongly. The increase is in the first place attributed to the enlargement of the Zwin reserve in 2019. An area of 120 hectares of farmland was converted back to saltmarsh and added to the reserve. The additional area has developed into a an ecologically well-functioning saltmarsh in the past five years. The large numbers of waders and ducks that feed there are testimony that the area has become very rich in food for these waterbirds. Besides adding 120 hectares to the already existing saltmarsh, the enlargement has also been beneficial for the ecological quality of that existing saltmarsh. The enlarged area was engineered in a way that about three times as much sea water enters the marine part of the Zwin reserve at every high tide than what was the case before the enlargement. This has also resulted in a stronger tidal influence in the saltmarsh that already existed before the enlargement. This has improved the saltmarsh habitat in that part of the Zwin reserve as well. The obvious increase in the number of waterbirds using the Zwin reserve since the enlargement of the saltmarsh area clearly shows that enlarging and improving natural habitats is vital for boosting populations of waterbirds. This is especially the case for areas like the Zwin nature reserve which are situated in a region like the Belgian coastal plain, with large human populations and the number of pressures on natural habitats that come with that.
Regular counting of the number of waterbirds present in the reserve is one of several monitoring schemes which run in the reserve in order to keep track of the state of nature in the reserve. This monitoring is important to keep track of the ongoing evolution of nature in general and birds in specific in the reserve.

Update on ‘Operation Stork’ – transmitters to track White Storks

The Zwin Nature Park is home to a breeding population of White Storks (Ciconia ciconia), with 9-16 pairs breeding in the period 2018-2023. This population is well monitored, with close surveying of the annual breeding performance and the annual ringing of all young storks in the nest. In 2019, a project was started in which a number of young storks were also fitted with a transmitter. The project, which runs in close cooperation with the Institute of Natural Sciences, is still continuing. A total of 22 storks have been tagged since 2019, with three to five new birds that were fitted every year. Seven of the tagged storks are still alive in March 2024.
The White Stork is an important flagship species for the Zwin Nature Park, being the result of an introduction project about 70 years ago and also being very popular among the visitors of the Park. The transmitter project is actively promoted. Through our website and social media we provide the public regular updates about the storks.
Five years of tracking storks has provided many interesting data. White Storks are long lived birds, which can live up to 40 years. Transmitter projects for this species need to be continued for many years in order to gather relevant data. It is no surprise that new insights are still coming up even if we are already more than five years into the project.
One of the surprises was that a good number of White Storks from our population still reach the Sahel zone in Western Africa as a wintering area. From ringing recoveries of young storks ringed in Belgium in the past decades the number of recoveries from Sub Sahara Africa could be counted on the fingers of one hand. In the past decades, White Storks from western Europe have increasingly started wintering in the Iberian Peninsula and Morocco and more recently also in France. Some birds even stopped migrating and stay for the winter at their northern breeding sites. It seemed that hardly any storks from our population still reached the Sahel if one looked only at the ringing recoveries. The results of our satellite project show that the story is more complicated. Of the storks that were fitted with a transmitter in the Zwin Nature Park, no less than six birds crossed the Sahara to winter in the Sahel. No small minority. Some of the birds reached Senegal and The Gambia. Others were further east, in Mali, Burkina Faso and Niger. The capacity to reach the far and more traditional wintering area in West Africa is clearly still in the migration genes of at least some of our storks. So far, only young White Storks in their first winter have crossed the Sahara. Birds that could be followed for more than one winter did not make the Sahara crossing in their second winter or later. Then they stayed further north, in Morocco or in Spain. Winter site fidelity soes not seem very strong for young storks. This pattern is also shown by other transmitter projects with White Storks in western Europe.
Landfills are obviously very important for our tagged storks. The birds find landfills all along their routes and wintering sites. From France through Spain to Morrocco: these sites are clearly a major attraction for White Storks looking for an easy and predictable food source. The storks that reach the West African Sahel as a wintering region are an exception to this. Whereas storks that winter at landfills usually have a restricted home range during winter, storks that winter in the Sahel cover vast areas, most likely in search of more natural food such as grasshoppers.
White Storks take several years to mature and to enter the breeding population. From their third calendar year the birds start looking for a nest site. This is often in the region where they were born but exceptions are not rare?. We are also seeing this in our transmitted storks. The birds that were tagged in the first years of the project have started to settle as breeders by now. One bird settled in the breeding colony within the Zwin Nature Park. The first serious breeding attempt in 2022 failed but in 2023 this bird managed to raise 1 chick, the first young stork to fledge from one of our transmitter tagged storks. Another transmitter stork settled as a breeder some kilometer from the Zwin Nature Park and bred
unsuccessfully for the first time in 2023. A third bird found a breeding place far from the Zwin, not far from the city of Caen in western France, about 350 kilometer from the Zwin.
More information about the White Stork transmitter project can be found on our website.

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