"Some of them (migratory birds) did not even leave because early winter was so warm and probably some of them first went south and came back," Lauri Haenninen, spokesman for environmental group Birdlife Finland, told AFP.

Skylarks, starlings and lapwings that usually head to warmer and more southerly climbs have been most affected by the mild weather.

"They should leave when the weather is colder," Haenninen said.

Blackbirds have started singing and ducks, also confused by the warm winter, are mating several months before they normally search for partners.

The unseasonally warm temperatures have also played havoc with the Nordic country's bear population. While those in the north of Finland have gone into hibernation as normal, bears in the milder south have emerged from their dens.