Migratory birds in the Wadden Sea

There is a difference between short-, medium- and long-distance migratory birds. Mallard and Lapwing are among the birds that migrate the shortest. They breed in Denmark, but if necessary, migrate to the coasts of Germany and the Netherlands for the winter. Eider and oystercatcher are among the medium-distance fliers, as they breed in Northern Europe and migrate to Western Europe. Finally, there are the absolute long-distance migrants. This group includes Arctic-breeding migratory birds such as the Icelandic red grouse, red knot and turnstone, all of which fly to tropical Africa for the winter. The Icelandic black grouse breeds in Siberia and flies to West Africa to spend the winter, stopping to feed in the Wadden Sea along the way. The birds lose a lot of body weight on the journey from Siberia to the Wadden Sea, but after spending a couple of weeks in the Wadden Sea eating protein-rich mussels, the Arctic char has doubled its weight and is ready for the final leg of the journey to Africa.

The birds' daily flights take place close to land, but when travelling, migratory birds soar to great heights. Smaller birds like the snow bunting fly one kilometer above the ground, while the common rail, red knot and lapwing migrate at an altitude of four to six kilometers. Larger birds such as geese, cranes and storks can fly as high as six to nine kilometers above the Earth's surface. To be able to fly at such high altitudes, birds must utilise oxygen better than species like humans, for example, by taking in oxygen themselves. Some bird species have up to nine lung pouches, but they can also use their hollow bones to store oxygen.

Migratory birds have an innate ability to find their way. This is known because migratory chicks are often left alone a few days after hatching and set off on migration without any training or help from their parents. How birds orientate themselves is far from clear. But the latest research shows that an area in the birds' brains contains neurons that can register the direction and strength of the magnetic field. This is likely to allow birds to correct their route if they go astray. However, birds' orientation is composed of many factors, as they can also orientate themselves by the sun, the stars, by land recognition and by their sense of smell.

With so many bird visitors, one might wonder if there is enough food for all of them? Fortunately, there is, and this is due to two main factors. The first is the enormous biomass of the Wadden Sea and the second is the different physiology of the birds. Even though birds compete for food, their species-specific physiology means that they can still share the food between them. For example, the length of a bird's legs and beak varies from species to species and affects where they find food. A bird such as a curlew has a very long beak and can find food deeper in the mudflats than an Icelandic kittiwake, whose beak is somewhat shorter. In this way, the birds are not in direct competition with each other.