Bimodality of spring migration of the Whooper Swan Cygnus cygnus in Finland
In the spring Whooper Swans migrate through Finland in two waves. The first wave moves about 350 km per day and the second seems to move slower, only 20-50 km per day. It is concluded that the first wave is formed by breeding birds and the second by non-breeders. The slower speed of the latter wave results in an arrival at the northern boreal region about one month later than the birds of the first wave. The first Whooper Swans arrive at Kuusamo ( 66°N-66° 33'N) in northern Finland in early April, some even in late March. Their arrival takes place many weeks before the breeding season starts. Pairs defend their pre-breeding habitat, slow flowing shallow rivers. Most birds stay, however, in small flocks of 11-40 birds. In Kuusamo the birds begin to visit breeding sites in late April. At that time the lakes andpeat lands are fully covered by ice and the nest mounds are under the snow. As the swans can localize the nest mound in such conditions, they must remember the site exactly from the previous season. Habitat selection must therefore have taken place in one o f the previous seasons when the site has shown its capacity as a breeding location.
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