The birds of the tidal areas

All over the world, many birds are found in the transition areas between land and sea. That is not so unusual, since coastal areas are often rich in food. The Wadden Sea is a good example. Ten million birds have been estimated in this area every year. Some of the bird species remain throughout the year, others come only to breed or to stay for the winter. Others stop over during migration.

International agreements have been made to protect such bird species during the conference of Ramsar. These agreements mean that when at least 1% of the world population of a specific bird species are found in a region, special regulations must be taken in those regions to protect these birds. There are many bird species in the wadden region that qualifiy for protection.

The Wadden Sea is particularly important for waders, such as the bar-tailed godwit. Nesting birds from the North Pole region migrate to Africa and return again in the summer. Some species fly distances of more than 5000 kilometers twice a year! The Wadden Sea lies halfway along the migration route. It is one of the few regions where the birds can find a sufficient amount of food to be strong enough for the continuation of their voyage. Therefore, protecting the Wadden Sea has worldwide significance.

From a study performed by the NIOZ, it appears that a large number of species are not doing well. Particularly birds that eat shellfish, such as the oystercatcher, the eider and the knot. Worm-eating bird species are increasing. The most plausible cause for these changes are the excessive shellfish fisheries in combination with a natural poor spat fall. It seems that the fished areas have become much more suitable for worms, and thereby more suitable for worm-eating birds.

There are five major groups of 'tidal-flat' birds: waders, geese, ducks, gulls and terns.

Waders

The waders are the most numerous. Many thousands of oystercatchers, redshanks, curlews, bar-tailed godwits and various species of sandpipers and plovers populate the Wadden Sea and the delta region for a shorter or longer period of time every year. They eat all sorts of benthic animals during low tide.

Geese

Geese such as Brent geese and barnacle geese are primarily winter guests in the coastal region of the North Sea, where they graze on the salt marshes. Together with the Dutch delta region, the Wadden Sea forms the most important overwintering area for the barnacle goose. In the second half of March, these geese migrate via the German tidal flats and the Baltic Sea to their breeding grounds near Nova Zembla.

Ducks

Duck species such as wigeons, common teal, shelducks and eiders also feel at home in the wadden region. Most ducks are carnivores and eat primarily benthic animals. The only vegetarian is the wigeon, which feeds on marsh plants and seaweed.

Gulls

The herring gull and the black-headed gull are the most obvious coastal birds along the North Sea shores. Gulls eat everything; they know how to profit from all kinds of human garbage sources to supplement their natural diet (fish, shellfish, crabs, starfish).However, their natural food must still make up an important part of their diet in order to stay strong.

Terns

Terns are typical summer guests in the North Sea region. They come to the North Sea to breed in April and May and they migrate again in the months of August and September to the south. The common tern and the Arctic tern breed in colonies spread out along the whole North Sea coast, while the sandwich tern is only found in the southern, sandy area.

Life in the coastal region

The environmental conditions for birds are different in the tidal zone than on land. Every bite of food for the birds contains salt. For eiders, it adds up to 50 grams a day. Kidneys cannot process so much salt. These birds excrete it in a different manner. They have glands above their eyes which remove the excess salt from the blood. Naturally, tidal zones are also wet areas. In order to stay warm and dry, these birds have a water-repellent plumage: the feathers fit perfectly around one another and are covered with a water-resistant oil, called tallow.

Eating during low tide

Benthic animals arre found in large numbers both in and on the tidal flats: sometimes more than a kilo 'meat' per square meter. This food is only available for most birds of the flats during low tide. The daily rhythm for most of the birds is therefore not determined by night and day, but by ebb and flood. During high tide, they fly in large swarms to places which do not flood, the so-called high-tide refuges. Only terns, diving duck species and cormorants can continue to look for food during high tide.

Competition in Dutch waters

In the Dutch section of the Wadden Sea, an average of around 700,000 tidal flat birds are present throughout the year. All of these birds need to eat. One would think that the one bird is taking away food from another one. However, the various species of birds living on the tidal flats have different means for finding food and therefore encounter little direct competition from one another.

People and birds of the tidal flats

The birds living in the coastal environment experience all kinds of human activities. Waders are very popular with people and the impressive swarms of birds are admired very much. Geese are sometimes combatted because they cause problems in agrarian crops.

Polluting their living area with oil and toxic materials, impoldering, fishery activities, mineral exploration and the growing recreation ('wad' hiking, aquatic sports) produce problems for them.

Shellfish fisheries, a traditional Dutch occupation

Many 'tidal-flat' birds eat shellfish; they cannot survive otherwise. However, man also likes to fish large amounts of shellfish. The mussel fisheries in the Netherlands is centuries old. The fishermen haul young mussels from the natural mussel banks in order to cultivate them on special parcels in the Oosterschelde and the Wadden Sea. Cockle fisheries is also an old traditional trade in the Netherlands, although it is only allowed to be caught by hand since 2005.

Disturbances

It is continually growing busier in the forage and rest areas for the birds of the tidal flats. All sorts of tourism create disturbances, especially in the summertime when many birds are molting and the number of tourists in the area is at a maximum. The birds are constantly taking flight which costs them a lot of energy, energy that is indispensable for renewing their plumage and for migration. The tendency for more and more ships to beach on the mud-flats where birds are foraging is becoming a serious problem, especially when the crew decides to wander around. It is precisely during low tide that the birds need to eat, so that these disturbances have harsh consequences. Since 2003, an explicit code of honour has been established for boaters in the Wadden Sea, whereby one attempts to avoid this kind of disturbance. Migrating birds are also sensitive to disturbances from military training, which disrupt part of the Wadden Sea.

Waders: changes since 1975

The international Wadden Sea attracts around ten to twelve million water fowl yearly. A large number of these birds are dependent upon the benthic fauna of the mud-flats and channels for food. Since 1975, the number of specimen of a large amount of bird species have been counted in scientific studies. The results show that since 1990-1991, shellfish-consuming birds such as eiders, oystercatchers, knot and herring gulls, have been declining.