The remarkable migratory patterns of the Arctic Tern

11 pictures

Interpolated geolocation tracks of 11 Arctic terns tracked from breeding colonies in Greenland (n = 10 birds) and Iceland (n = 1 bird). Green = autumn (postbreeding) migration (August/November), red = winter range (December/March), and yellow = spring (return) migration (April/May). Two southbound migration routes were adopted in the South Atlantic, either (A) West African coast (n = 7 birds) or (B) Brazilian coast. Dotted lines link locations during the equinoxes

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It’s official: the Arctic tern has the longest migration of any animal in the world. The Arctic Tern Migration Project recently discovered that the tern flies over 70,000 kilometers (43,496 miles) annually, from its breeding grounds in the Arctic to its winter quarters in the Antarctic. That distance is more than twice what was previously estimated. Over the lifetime of one bird, it travels approximately 2.4 million kilometers, the equivalent of three trips to the moon and back. For a bird that weighs just over 100 grams (3.5 ounces), that’s fairly impressive.

Mapping the route

The data was obtained by placing miniature archival light loggers (Also known as geolocators) on a number of breeding Arctic terns in 2007 - 50 birds in Greenland, and 20 in Iceland. Light logging has been around for over a decade, but until recently the loggers were too heavy to be used on smaller birds. Now, thanks to cutting-edge technology, the loggers are small and light enough to be attached to the terns. The loggers work by recording and storing ambient light intensity, which in turn reveals information on sunrise and sunset. When this data is combined with time recordings, two geographical positions per day can be calculated, which adds up to a record of the entire migration route.

The data retrieval process

While light loggers are much lighter and cheaper than conventional satellite transmitters, they don’t transmit their data, so the only way it can be retrieved is by recapturing each bird and removing the device. Fortunately for this study, terns will often nest at the same location two years in a row. As it turned out in 2008, the researchers were able to retrieve ten loggers in Greenland, and one in Iceland. Some of the tagged birds couldn’t be recaptured, and others presumably shifted colonies or simply skipped that breeding season altogether.

Some interesting surprises

The southern autumn migration was found to be longer in both time and distance, due to the birds’ stopping for approximately 25 days at an open-ocean “hot-spot” near the North Atlantic Ridge. The site is in an area where cold, highly-productive northern water meets warmer, less-productive southern water. From there, most of the birds followed the coast of West Africa, although some chose to go along the coast of South America. When they headed back north in the spring, they were found to take a much faster route, spending as little time as possible in the tropic and temperate zones. This makes sense, as warmer waters are less productive, and would therefor provide the terns with less food. They were also found to follow the prevailing wind systems, instead of taking a more direct but more strenuous route north.

All in all, pretty fascinating stuff. “This study on seabird migration has given us an incredibly detailed insight into how long-distance migrants behave at times of the year when it’s normally impossible for us to follow them” said Carsten Egavang, of the Greenland Institute of Natural Resources.

All photographs courtesy Carsten Egevang/ARC-PIC.COM

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