Oxford, UK - The world's largest falcon, the fast, taloned gyrfalcon, is a secret seabird, scientists have discovered.
Gyrfalcons living in the high Arctic overwinter out at sea, spending long periods living and hunting on pack ice.
It is the first time any falcon species has been found regularly living at sea.
The birds likely rest on the ice and hunt other seabirds such as gulls and guillemots, over what appears to be one of the largest winter ranges yet documented for any raptor.
"I was very surprised by this finding," said ornithologist Kurt Burnham who made the discovery. "These birds are not moving between land masses, but actually using the ice floes or pack ice as winter habitat for extended periods of time."
"Previously, all species of falcon were considered to be land-based birds."
Dr Burnham of the High Arctic Institute, Illinois US and the University of Oxford, UK, together with colleague Professor Ian Newton of Centre for Ecology and Hydrology in Oxford, studied the seasonal movements of gyrfalcons (Falco rusticolus) by tagging 48 birds with radio transmitters.
This allowed them to track the movements of the birds living in three areas of Greenland: Thule in the northwest, Kangerlussuaq in the west, and Scoresbysund in the east.
They found a huge amount of variation between individual birds.
Birds living on the west coast had winter home-range sizes of between 400-6,600km square kilometres.
Those on the east coast ranged far more widely, covering between 27,000-64,000 square kilometres. Some of these had no obvious winter home ranges and travelled continuously during the non-breeding period, spending up to 40 consecutive days at sea.
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During the winter one juvenile female travelled more than 4,500km over 200 days, spending over half that time over the ocean between Greenland and Iceland.
"Others had observed gyrfalcons sitting on icebergs and over the ocean but it was always assumed they were only over the ocean for a short period to hunt and then flew back to land," explained Dr Burnham.
"We show that some gyrfalcons actually spend large amounts of time living and hunting over the ocean and pack ice, and that sea ice is actually an important and previously unknown winter habitat for [them]."
Gyrfalcons in Iceland and the low Arctic are thought to be residents, staying in the same location all year. In the high Arctic, gyrfalcons are known to be migratory, but little was known about where they overwintered.
A few other falcons also cross the ocean when migrating, such as peregrine falcons crossing the Gulf of Mexico. But the gyrfalcons are doing more than just moving between land masses in this way.
A gyrfalcon surveys its range "These individuals are likely resting on icebergs and ice floes and hunting the seabirds, such as black guillemots, dovekies, and species of gull, that are using the same habitat," said Dr Burnham.
The size of the bird's home range over the ocean also surprised the scientists.
Dr Burnham told the BBC: "The other main finding is the huge area that some of these individuals are using.
"In the big picture this shows how adaptable and mobile gyrfalcons have to be in order to survive and reproduce in the harsh arctic environment they live in.
"Food or prey can be scarce during the winter, and having the ability to travel somewhat long distances on a daily basis and spend long periods of time away from land increases their chance of survival.
The research, he added, emphasises how specialised many Arctic species are, in order to survive in an extremely difficult and inhospitable environment.
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