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There are nearly 10,000 known bird species around the

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world and roughly half of these

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migrate.

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Yes, from songbirds to

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seabirds, riverbirds to

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raptors. So for this week's podcast

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on English.com we ask why

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they do it and how.

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Yeah, it's an amazing thing. There

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are birds that fly from the North

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Pole to the South Pole a distance

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of 90,000km every year.

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I mean that can't possibly be true, can

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it?

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It is, but never mind the distance.

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Some of them cross over the Himalayas

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reaching altitudes of up to 7,000m. Yeah,

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the record for the longest

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non-stop flight of any bird

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is over 12,000km. It

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took 11 days. So no

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stopping to sleep and nothing to

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eat or drink.

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Yes, but it's not

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only flying. Some penguins,

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obviously birds, are known to trek

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13,000km every

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year. Yeah,

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some amazing figures there. So we ask

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the first question, why? Yes,

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obviously there are two main reasons. The

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first is food and the other

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one is finding suitable places

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to nest. Now a good example, Richard,

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is the ospreys we see in

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Scotland, right? Yes. Now

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they live throughout the winter in West

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Africa. The weather is warm and

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there's plenty of fish for them.

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Yes, now migration is

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generally in a north-south direction.

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So places like Scotland have

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more daylight hours in the summer, up

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to 18 hours, compared to

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only 12 hours in West Africa.

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So there's more opportunity for them, six

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hours more, to catch fish.

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fish for themselves and

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they're young. So that's why they

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fly north in spring. And then

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when it gets cold they return down south.

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Yes, exactly. And of course the forests

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in Scotland are perfect for nesting.

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And in Scotland it's

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relatively predator free.

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OK, so that's why, Richard.

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But how? Well, the ospreys

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are a good example here too. Come

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the end of the summer, the female osprey

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flies down south on her own.

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The male stays behind

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and continues to feed the young

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and then he leaves to go south. Finally,

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the young ones are left

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to begin their migration on their

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own.

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You see, on their own, Richard. Little

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ones on their own for the first time.

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How? And the

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answer is also strange

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because in fact nobody really

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knows. Once they've arrived in Africa,

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the ospreys, the young ones, they

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don't return for the first three

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or so summers. But they'll return

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to Scotland.

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Lots that we don't know. But

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what do we know? Well, it seems

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that birds typically follow established

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flyways. Most stop and

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refuel along the way. It seems

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they use visual clues.

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Right. So, OK, so during the day,

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the sun and geographic

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features like rivers.

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Yes, and also at night they use

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the position of the stars and the moon.

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But also it seems birds

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have a magnetic compass

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inside them to navigate. They have

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a sort of internal GPS

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system that prevents them getting

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lost. We know this from knowledge

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gained from tagged birds and

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actually those that have GPS tags

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on them.

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Yeah. Our Scottish ospreys

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travel by day. A lot travel

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by night.

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and they use the thermals to gain

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height over land. It takes

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them about 45 days

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to get to West Africa. Absolutely

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amazing. But however

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they do it, migration is

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a truly incredible feat.