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They migrate thousands of kilometres across the sea without getting lost.

The Arctic tern, for instance, spends summer in the UK, then flies to the Antarctic for the winter.

Yet, scientists are still unsure exactly how birds perform such extreme feats of migration, arriving in the right place every year.

According to new research, smell plays a key role when birds are navigating long distances over the ocean.

Researchers from the universities of Oxford, Barcelona and Pisa temporarily removed seabirds' sense of smell before tracking their movements.

They found the birds could navigate normally over land, but appeared to lose their bearings over the sea.

This suggests that they use a map of smells to find their way when there are no visual cues.

Previous experiments had suggested that removing birds' sense of smell impairs homing ability. However, some had questioned whether sensory deprivation might impair some other function, such as the ability to search for food.

''Our new study eliminates these objections, meaning it will be very difficult in future to argue that olfaction is not involved in long-distance oceanic navigation in birds,'' said study researcher Oliver Padget of Oxford University's Department of Zoology.

He said seabirds were among "nature's greatest navigators", finding their way over huge distances.

The researchers studied 30 Scopoli's shearwaters living off the coast of Menorca. The birds nest in the Mediterranean, but spend the non-breeding season in the Atlantic, including areas off the west coast of Africa and the east coast of Brazil.

Some of the birds were made to temporarily lose their sense of smell through nasal irrigation with zinc sulphate; another group carried small magnets; and a third group acted as a control.

Tiny GPS devices were attached to the birds as they nested and incubated eggs on the rocky coastline.

The birds were then tracked as they went about their daily business.

The birds with no sense of smell made successful foraging trips to the Catalan coast and other distant sites. However, they had problems navigating when they were out of sight of land.

When they approached land, their orientation improved, suggesting birds use an olfactory map when there are no visual landmarks.

The question of how birds navigate and perform long-distance migrations has long been of scientific interest.

Birds' sense of the Earth's magnetic field and their olfactory abilities are among the factors thought to play a role.

The research is published in the journal Scientific Reports.

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