PARIS, Oct 21 (AFP) - European birdwatchers are scratching their heads over why a particular species of bird, the booted eagle, is migrating north this winter instead of the balmier south.

According to the Bird Protection League (LPO), nearly 1,000 of the rare eagles have been spotted in southern France over the past two weeks - more than 30 times the normal number - after reversing their normal September movement which should see them head to Africa and India.

The bizarre phenomenon has never been seen before, the LPO stressed. Speculation that the birds, which have a wingspan of up to 1.3 metres (4.3 feet), might have been affected by weather disturbances linked to global warming has butted up against the fact that no other species appears to be having the same navigation trouble.

Normally, the booted eagle (Hieraaetus pennatus) spends the warmer months of the northern hemisphere breeding in southern Europe and parts of north Africa.

Come September, the raptor flies south to escape the chill, with a migratory peak around the middle of the month.

The eagles, which mate for life, are listed as an endangered species.