Over 99% of the birds that spend summer in Finland move away over the winter and a large part of them have already flown south, according to ornithologist Andreas Lindén, a senior researcher and associate professor at the Novia University of Applied Sciences.
Last week's cold snap, the first of the winter, didn't cause mass deaths among birds.
"Migratory birds have gone and water birds that are completely dependent on water have either moved south or gathered in areas where there is open water," Lindén explains. "A lot of thrushes and finches moved enough away then. Even in the middle of the worst snowstorm many birds were seen flying south."
"The birds that absolute have to move south are gone," says Lindén.
Some birds common in Finland, however, are semi-migratory species. In such species some of the individuals head south for the winter, while some of them decide to stay.
Gender and age can be factors, but the availability of food is crucial. Last week's snow cover could have convinced them to go.
"We can imagine that some individuals that maybe considered spending the winter now have flown away."