Bird migration
| Bird migration has been studied since ancient days. Many birds found in the North Sea region are migratory birds. Waders, ducks and geese that breed in more northerly regions, migrate in late summer to the wadden region. Some of them will overwinter here, others migrate further south, after having filled their bellies on the tidal flats. The trip back to the breeding area resumes in the early spring. |
Migrating birds orient themselves during migration during the day using the position of the sun and at night using the position of the stars. Scientists discovered this by using mirrors to manipulate the position of the sun that was visible for them. The birds reacted by choosing a different direction to migrate. Birds also use the ground to orient, particularly using rivers and canals running in a north-south direction. In addition, the earth's magnetic field and polarized light play a role. In fact, German scientists have discovered how birds use the earth's magnetic field to determine their flight route. Birds have special nerve cells in their eyes that are sensitive to magnetic directions.
The reason for bird migration is the lack of food in the breeding grounds during the cold season. Before the birds are confronted with too little to eat, they leave for warmer regions. Likewise, the moment to return has been determined genetically. During the Middle Ages, one used to think that the birds changed to a different species in the winter, such as the cuckoo turning into a sparrowhawk. Of that the barn swallow hid in the mud. Some Middle Age birds flew to the moon or changed into fish.
The wind is the most important factor determing the altitude chosen by most birds when migrating. Temperature and humidity are less important. In northwestern Europe, the altitude is usually limited to 4 kilometers. The average altitude is between 2-3 kilometers. Migrating birds profit the most when flying with a wind velocity that matches their own flying speed. If the wind blows stronger than their own speed, birds will often stay on the ground to avoid loosing control.
Predicting bird migration improving all the time
In Leeuwarden, a radar from the air force keeps track of where birds fly and how they migrate. The radar can observe birds up to dozens of kilometers away. In 2007, the air force was using a model called BAM (Bird Avoidance Model), which combines weather maps, radar echos and geographic information to prevent bird collisions with airplanes. This model can predict the intensity of a migration days in advance.
The European Space Association ESA wants to cover the entire European region with a radar network to measure bird migration. At the moment, predictions are made based upon data and can only be used locally. ESA is developing a variation to the BAM, called Avian Alert, and uses data from Air Forces in Germany, Denmark, France and Belgium. Several Dutch biologists are involved with the making the calculations using geographic information and historical counting data from SOVON.
The biologists first studied data from 28 weather factors and measured night migration via the BAM for several years to determine which factors have the most influence on night migration. This turned out to be wind (as opposed to air pressure, temperature, etc). Their goal is to be able to predict bird migration everywhere, based upon theory.
At the moment, when compared to Danish, German and Swedish models, all models are very dependent upon local measurements. Weather and bird counts in the Netherlands is fine for predicting night bird migration in the Netherlands, but not in Germany, for example.
To reach their goal, the biologists believe that each individual country must make their own model based upon local measurements. Then the Avian Alert will be able to combine these models and start making global predictions over the borders. Even after such an accomplishment, Avian Alert does not take migration occurring at extremely high or low elevations into consideration and recognizing the species of bird is still impossible.
Migration of land birds
Land birds migrate at high elevations and don’t like flying over large bodies of water. The best place to see them is by sea straits, where the necessary crossing is the narrowest and is used the most often. Many migratory birds can be observed every year in the north of Texel (by De Cocksdorp). In the autumn, one can literally picked them off of the beach and bushes, where the birds catch their breath after the wide crossing. This is also where they gather in groups in the spring, using it as a pit stop before re-crossing the North Sea.
Refuelling on the flats
Migratory birds must keep to a strict travelling schedule. Should they arrive too late at their nesting grounds, it could mean that the young are not mature enough by the end of the summer to make the long voyage south. If they arrive too early, everything is covered in snow and ice. Furthermore, it is of vital importance to bring along enough fuel. They build up their reserves in the form of fatty layers and heavy muscles in areas rich in food, such as the Wadden Sea. In the spring, these reserves are used to fly north as well as to survive the first period in the nesting area and to lay eggs. This makes the Wadden Sea not just an arbitrary place along the migration route, but an essential intermediate station, where millions of birds ‘tank up’.
A good example is the red knot. This bird nests in Greenland, Canada and Siberia. The Greenland and Canadian birds molt and overwinter in Western Europe; most of them stay in the Wadden Sea. The Siberian knots also visit the Wadden Sea but migrate further to the coastal regions of West Africa, where they spend the winter. When the knots return to the Wadden Sea in May, they have used up all of their fatty reserves. Here they eat until they practically burst in a very short time before taking off for the second part of the trip to the Siberian tundra.
The Arctic tern is also a remarkable migratory bird. Of all the terns, this species migrates the furthest south to overwinter, down to the ice of the South Pole. It returns in the spring to its nesting grounds in the North Sea region and further northwards, which totals a yearly trip of around 35,000 kilometers!
The spring migration does not need to follow the same route as the autumn migration. For example, common terns migrate to Africa in the autumn. The birds fly along the coast because the most sardines can be caught there. In the spring, the sardines are generally found in open sea. The common terns adapt their routing so that they can still catch lots of sardines along the way.
Seabird migration
Not all birds migrate via the Wadden Sea. Seabirds usually migrate without stopping to forage. They eat namely while they fly. The birds tend to fly low, in comparison with land birds that fly high in the sky. When they see a suitable prey, they can hunt it without too much delay.
Seabirds build up much fewer fatty reserves for migrating than land birds since they forage along the way. The advantage is that they can fly lighter, which costs less energy.
With seabirds, it is more difficult to establish the route than with land birds. Not every movement of a seabird is part of migration. In general, seabirds wander a lot over the seas. After brooding, they choose to depart again. Some species (such as the manx shearwater) consciously leave to a specific destination (in this case, Brazil); others wander around, just like they did before the nesting season (such as the albatross).
World record bird migration
A young common tern has the world record for bird migration when it departed as a ringed chick from central Finland on June 30 1996. It was found on January 24 1997 in Gippsland Australia: a trip of 22,500 kilometers. One assumes that this bird started to follow the normal route for common terns, from Finland to the North Sea and further along the Atlantic coast down to South Africa. Somewhere along the way, the bird got caught in a storm and ended up in Australia. It had to have flown 200 kilometers a day.
Winter guests
For many birds which breed in northern regions, the climate in the wadden region is mild enough to spend the winter. The enormous flights of barnacle geese populating the fields in the Lauwersmeer region and on Schiermonnikoog are spectacular sights. Large groups of tundra swans, bean geese and brent geese forage on the northern polders on Texel. Furthermore, many duck species such as mergansers, gadwalls, smews, northern pintails, wigeons and common pochards often choose the tidal flats as a winter residence.
Groups of fieldfares and redwings, thrush species which were highly prized by hunters in earlier days, swarm during the winter in the cultivated areas and dune regions. One bird of prey which stays in the wadden region in the winter is the rough-legged buzzard.
International migration bird counts
In order to know how many migrating birds there are, 34 countries participate on a European scale in counting birds during migration season by tens of thousands of bird-lovers. The organization Euro Birdwatch organizes these counts, which is coordinated in the Netherlands by the Dutch Bird Society (Vogelbescherming). In total, there were 259 species and 788,620 birds counted in 2005.
Green lights
Artificial light disrupts flying habits of migrating birds. Drilling platforms illuminated with traditional 'white' light influences the birds. Sometimes it is positive, illuminating a resting and gathering place for gulls, eiders and common scoters. Platforms in general also tend to attract fish and other marine fauna, which the birds consume. However, there are also negative aspects, particularly for migrating songbirds, waders and ducks. Night migration for these species is very important. The darkness protects them from predators and the weather is generally more stable than during the day. But when these birds see illuminated objects along the way, they tend to waste a large amount of precious energy flying around them.
After experimenting with turning off the lights, the birds left the area and no other problems could be detected. A study was then made with different colors of light to see how that would affect the birds. Results showed that the red end of the spectrum disturbs the geomagnetic compass in the animals. Lights were developed with a limited amount of red and the number of birds around the platforms at night decreased by 80 to 90%. By applying these green lights on all of the platforms in the North Sea, the researchers expect a decrease in the number of disrupted birds from around 6 million to 600,000 birds.
Yacht harbours in the wadden regions are also very illuminated at night. An experiment with green lighting was performed in the harbour on Ameland, with positive results. The development is still in the experimental stage. In addition, it is unknown what the effect of green light is on other marine animals, such as seals and fish. However if green illumination radiates a positive sustainable situation, it is very likely that harbours along the Dutch coast as well as installations at sea will switch their light fittings.
Source: de Vleet, Ecomare
