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Epic cross-hemisphere songbird migration revealed

Thu, 02/16/2012 - 12:24am
Natural Environment Research Council

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Epic cross-hemisphere songbird migration revealed

16 February 2012, by Tamera Jones

Scientists have used miniature tracking devices to reveal the epic 30,000-kilometre (18,641-mile) migrations of a small songbird called the northern wheatear.

Northern wheatear

Northern wheatear.

The researchers found that the birds take between two and three months to get from their breeding grounds in the Arctic to sub-Saharan Africa. And even though they weigh just 25 grams – slightly more than a robin – they cover an incredible 290 kilometres (180 miles) every day.

'We had an idea birds from Alaska migrate as far away as Africa, because there had been sightings in the Pakistan, Iran and the Arabian Peninsula. But we were astonished to find that's the case,' says Dr Heiko Schmaljohann from the Institute of Avian Research in Germany, a member of the study team.

'It's the only songbird that links the Arctic with Africa. The vegetation in the two is completely different,' he adds.

The findings are published in the Royal Society journal Biology Letters.

'It's the only songbird that links the Arctic with Africa. The vegetation in the two is completely different.'

Dr Heiko Schmaljohann, Institute of Avian Research

Scientists have long known that northern wheatears have one of the largest ranges of any songbird in the world – they breed over a huge area of the Arctic, in eastern Canada, Greenland, Eurasia and Alaska.

But the birds are made up of two separate subspecies that appear never to meet. One subspecies breeds in eastern Canada, Greenland and Iceland, while the other uses Eurasia and Alaska.

And despite sporadic reports of sightings in North America, nobody has ever seen either subspecies spending the winter in the Americas, even though this might seem the obvious choice for Alaskan wheatears. This suggests that all wheatears from across the Arctic somehow make their way to Africa.

To test these ideas, Schmaljohann and colleagues from Germany, Canada and the UK decided to use miniature light-recording devices called geolocators to track wheatears migrating to Africa from the Arctic.

These devices – developed at the British Antarctic Survey – weigh just 1.2 grams and, including attachments, make up about six per cent of an adult bird's body weight. They record light intensity; when this data is fed into a computer program, scientists can figure out when and where the birds travelled.

Schmaljohann, his colleague Franz Bairlein – also from the Institute of Avian Research – and German and Canadian colleagues went to Alaska and Canada to fit 46 birds with the geolocators.

The team spent months testing the devices to make sure they didn't affect the birds' flight. And when they tested them on European wheatears, they managed to retrieve around half of the geolocators. But it was a different story in the Arctic.

'The site fidelity of the Arctic birds is much lower,' Schmaljohann explains. This means only five of the 30 Alaskan wheatears they tagged returned. And just two of the 16 birds tagged in eastern Canada returned. Of those seven, the researchers only managed to retrieve data from four devices.

Despite this, the team got the results they were looking for.

They found that Alaskan wheatears fly overland to eastern Africa to spend the winter there, covering about 13,500 kilometres (8388 miles). But birds from eastern Canada and Greenland cover around 3500 kilometres (2174 miles) getting to western Europe, grappling with the stormy north Atlantic on the way, before making their way to Africa.

'The Alaskan birds fly further to get to Africa, but birds from eastern Canada have the north Atlantic to contend with. It's a huge barrier and results in many casualties,' says Schmaljohann. 'To make the trip, they have to double their body mass up to around 50 grams, because they need a lot of fuel.'

'We might think it's a long way, but from the birds' point of view, it's probably no big deal. It shows that we completely underestimated the capabilities of these birds,' he adds.

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