Migratory birds that fly across the continents
West Sweden's migratory birds do something that many people dream of doing: they leave Sweden during the winter half of the year and stay in warmer latitudes. But why do they do this? Welcome to an airborne odyssey, from north to south and back again.
Each summer around 70 million pairs of birds nest in our country. Around 60 million leave during the winter. In the autumn around 500 million birds fly south over southern Sweden – far more than those returning after nesting here. A large part of this huge number of migratory birds are yearlings on their first long journey.
A swallow weighing all of nineteen grams, or a willow warbler of eight grams hatched in a nest somewhere on the west coast of Sweden, undertake voyages of up to ten thousand kilometres over the vast Western Sahara down to the south of Africa. Another large group of migratory birds come from nesting sites way north in the Russian tundra or Greenland, and are only passing the west coast on their way to and from their wintering areas on the European continent or in Africa.
From north to south and back again
The ability to fly has made it possible for birds to be highly efficient in terms of available food resources. There is plenty of food in the north during the summer half of the year. By flying away from the south, migratory birds do not need to compete for food with stationary southern birds. Full parents have more energy to feed larger broods than hungry parents.
During the winter half of the year there is little food to be had in the far north, and so it is time to head south again. Can those extra fledglings really compensate for all the trials and hardship of migrating twice a year?
Clearly, the benefits of migration outweigh the drawbacks; otherwise the vast majority of our birds would not do it. But how can it be worthwhile? A small mouse with the same weight as a swallow runs around using four times less energy than the swallow's long flight. It is expensive to travel by air - but it pays off!
Roughly speaking, a bird flies ten times faster than a mammal runs. The transport cost, in terms of energy consumption divided by distance, is three times higher for a running mouse than a flying swallow.
The speed and economy of flying means that despite all the hardships of migration, it is still a good deal for birds to reproduce in the north and spend winters in the south.
Leavers and stayers
The willow warbler and the chaffinch are the most common migratory birds in West Sweden. The robin is number three, but is not an out-and-out migratory bird. Many birds choose a simpler option: they overwinter in people's gardens where they are hopefully provided with breakfast and dinner, preferably in the form of seeds, tallow balls, raisins, apples or oatmeal.
Do you want to know more?
Leif Lithander, curator at the Gothenburg Museum of Natural History.
Phone: 010-441 42 46