Planet Earth Online

Arctic terns top long-distance migration list

12 January 2010, by Tamera Jones

As Arctic terns can live for 30 years, this is like making three trips to the moon and back.

Although scientists have long suspected that Arctic terns top the list of long-distance migratory birds, previous estimates suggested they travelled an average of 40,000 kilometres every year.

Now, new technology shows that they travel double this distance.

'This is a mind-boggling achievement for a bird that weighs just over 100 grams,' says the lead author of the research, Dr Carsten Egevang from the Greenland Institute of Natural Resources.

Egevang and colleagues from the British Antarctic Survey, Greenland, Denmark, the US and Iceland describe in the journal Proceedings of the National Academy of Sciences how they tracked the birds using a tiny instrument called a geolocator.

They attached the devices to 50 Arctic terns' leg rings towards the end of the breeding season on Greenland in July 2007. Another 20 instruments were attached to birds breeding on Iceland in June 2007.

The geolocators weigh just 1.4 grams and, including attachments, make up around 1.9 per cent of an adult'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.

The following year the team managed to retrieve ten of the logging devices from the Greenland birds and one logger from the Iceland colony.

As well as confirming their wintering grounds in Antarctica, the researchers discovered that after leaving Greenland and Iceland in August, the terns stop over in the North Atlantic for around a month en route to Antarctica.

'This part of the Atlantic is a region of high marine productivity so it's clear they're using the area to re-fuel before crossing less productive parts of the Atlantic Ocean,' explains Dr Richard Phillips of the British Antarctic Survey, co-author of the paper.

While birds from the same colony followed either the African or South American coastlines, all of the birds followed the same route home. This means that birds from different breeding colonies mix when they're in the Antarctic.

Previous researchers have suggested that Arctic terns circumnavigate the Antarctic continent in early spring before starting their migration back to the Arctic. But Egevang and his colleagues found that the birds spent the Antarctic summer - December to March - in the Atlantic region of the Southern Ocean and didn't venture much further afield.

'This part of the Southern Ocean is particularly productive and supports higher numbers of Antarctic krill, which is a favourite food for terns,' explains Phillips.

Come April, all the birds embarked on the return journey to the Arctic, taking an anti-clockwise route around the South Atlantic and a clockwise route around the North Atlantic.

'It looks like these incredible birds are exploiting the prevailing global wind systems in the Atlantic, which go clockwise in the north and counter clockwise in the south, making their trip north that much easier. Interestingly this is the same strategy used by Manx and Cory's shearwaters,' says Phillips.

This means the terns took a mere 40 days to travel the huge average distance of 24,270 kilometres from Antarctica to Greenland.

Overall, the return journey to the Arctic took less than half the time it took for the terns to travel south, even though the northbound trip is three quarters the length of the southbound trip - 25,700 kilometres compared with 34,600 kilometres.

By May they reached the same stopover in the North Atlantic they used on their route south.

'The use of these devices on seabirds is not only revolutionising our understanding of migration patterns, but the resulting data on distribution also help address the requirement to identify important biological hotspots,' Phillips adds.

Keywords: Antarctic, Arctic, Atlantic Ocean, Oceans, Polar,

Interesting? Spread the word using the 'share' menu on the top right.

• Home
• Back to top