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Hello, for millennia, bird migration was a complete mystery to humans.

Today, while what we know is remarkable, much of blood mystery remains.

There was an idea in ancient Greece that birds turned into fishes when they were not around.

In Age of Folk laws, some were thought to turn into barnacles, others to hymenate in cliffs or at the bottom of lakes.

And perhaps those ideas are less extraordinary than what we now know.

For example, the birds weighing less than a cup of water can fly across oceans non-stop from New Zealand to Alaska, breed and return.

How birds know where they're going is not yet fully understood, but may include some combination of internal clocks,

of ways of detecting magnetic fields and of heightened sense of smell.

With me to discuss bird migration are Barbara Helm, reader at the Institute of Biodeiversity, Animal Health and Comparative Medicine at the University of Glasgow.

Tim Gilford, Professor of Animal Behabia and Tutorial Fellow of Zoology at Burton College Oxford, and Richard Hollen, Senior Lecturer and Animal Cognition at Banger University.

Tim Gilford, can you define migration for us and it is lonely for birds? Can you give an overall view of migration?

Yes, well I think migration means different things to different people, but the sort of classical definition of migration would be something which a bird for example or a turtle which makes a very long journey repeatedly from one season to the next, from one year to the next,

between a breeding site in one location and an overwintering site somewhere else on the planet. So it sees very long distance, repeated return movements which are usually seasonal between breeding and overwintering, the iconic definition of migration.

People do refer to migration of plankton and fish on a daily basis in the marine water column, up towards the surface at night and down deeper during the day and some people would regard that as migration.

So it's different things to different people, but it always involves this kind of cyclical, surprising movement.

So birds are not alone in this, that's what I'm trying to establish.

Absolutely right, some of the greatest migratory species on the planet are turtles for example or whales.

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