The Arctic Tern - extreme migration from pole to pole


Arctic tern flying
The Arctic tern (Sterna paradisaea) is an elegant flyer performing an annual long-distance migration between the Arctic and Antarctica.

Back in 2010, new knowledge on the migration of the Arctic tern was published in the scientific journal PNAS. The study was part of my PhD focusing on the Arctic tern in Greenland and revealed longest annual migration ever recorded in any animal.


The annual roundtrip of the Arctic tern is more than 71,000 km. From ringing recoveries, it is known that Arctic terns can reach an age of more than 30 years. The total distance flown in a tern’s life may exceed 2,4 million km., which is equivalent to 3 return journeys to the moon. A mind-blowing achievement by a bird with a body mass of little over 100 grams.


The Arctic tern breeds in summer under the Arctic sun with 24-hours of day light. At the winter quarters, the Arctic tern again take advantage of long days under the Antarctic summer (November to January). This makes the Arctic tern the animal that probably receives most day light in the World.




Results

This study on Arctic tern migration revealed new information on the longest migration ever recorded in any animal. Impressive distances The average roundtrip distance from Greenland/Iceland to the Weddell Sea (Antarctica) and back again, was of 70,900 km, with a range of 59,500 to 81,600 km. This is nearly twice the distance generally cited for the annual Arctic tern migration.

As Arctic terns can live for over 30 years, the total distance flown in a tern’s lifetime may exceed 2.4 million km, that’s equivalent to around 3 return journeys to the Moon!

The southbound migration in the autumn was of longer duration and involved greater distances compared to the northbound spring migration. At approx. 40° S the birds paused their southbound migration for a period and moved along the Polar front in both eastern and western directions. One bird travelled as far east as 106º E, well into the Indian Ocean, at approximately the same longitude as Australia.


Arctic tern migration routes
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). Dotted lines link locations during the equinoxes

At-sea hot-spot The study also revealed several interesting patterns in the migration of the Arctic tern. The birds spent considerably longer periods of time in some areas at-sea compared with other areas. From the data, it was possible to identify a previously unknown at-sea stop-over site for Arctic terns in the North Atlantic Ocean. The birds stopped their southbound migration and spent an average of 25 days at the western slope of the mid-North Atlantic Ridge between 41-53° N and 27-41 W. This oceanic stop-over site was located at the junction between cold, highly productive northern water and warmer, less-productive southern water, and was characterised by high eddy variability.


Two migration routes Seven of the birds chose a path that followed the coast of West Africa, as expected from ringing recoveries, but four individuals crossed the Atlantic just north of the Equator and went south along the South American continent. Although these four birds exhibited a different migration route in autumn, whey shared the wintering site, northbound migration pattern, and timing of migration with the other seven birds.


Pole to pole migration by the Arctic tern
Simplified figure showing migration patterns of the Arctic tern, from the breeding sites in Greenland and Iceland to the winter grounds at Antarctica. After initiating the southbound migration (yellow line) the birds paused their migration in the central part of the North Atlantic (small circle) for almost a month before they continue towards the wintering sites at Antarctica (large circle). In spring, the northbound migration (white line) is conducted more than twice as fast in a gigantic “S” shaped pattern through the Atlantic Ocean. Areas particular rich in biological productivity are indicated by yellow and green colours.

Uneven migration speeds While the migration south to the winter quarters was conducted over a period of three months (average 93 days), the northbound migration back to the breeding sites was much faster. The terns covered the 25,700 km from the winter site north to 60º N in an average of 40 days. This last long leg was conducted with average daily distances of 520 km – with some individuals flying up to 670 km per day. The general pattern of Arctic tern migration indicates that the birds spent as little time as possible in the tropic and temperate zones (between approx. 40º N and 40º S) on both sides of the Equator.

Correlation with marine productivity The migration patterns of the Arctic tern correspond well with our knowledge of marine productivity. From space, it is possible to monitor the concentration of chlorophyll (which gives an indication of high productivity in the food web) originating from algae blooms in the world oceans and map the areas of significant biological importance. In general, tropical waters have lower productivity than sub-polar and polar waters. This study on Arctic tern migration shows that the birds utilised areas with high concentrations of chlorophyll, such as the previously unknown oceanic hotspot in the central part of the North Atlantic and along the polar front at the southern hemisphere.


Correlation with prevailing winds systems After having spent the winter in the Weddell Sea, the Arctic terns started their northbound migration, but did not fly in a straight line towards their northern breeding grounds. At the onset of the spring migration the birds all showed a northeast bearing until the coast of Namibia, where they altered the bearing to northwest. After having crossed the Equator, the birds continued northwest almost as far as the Caribbean, before they again altered their bearing towards their breeding grounds to the northeast. The northbound track forms a gigantic “S” up through the Atlantic Ocean. This pattern corresponds well with the prevailing global wind systems, being clockwise in the North Atlantic, and counter-clockwise in the South Atlantic.

Distinct winter site Several researchers have suggested that the Arctic tern may travel around the Antarctic Continent during (boreal) winter. The results of our study show that this is not the case. In fact, although the birds were distributed over large areas (ranging from the coast of South America to almost as far as Australia) prior to the onset of winter, they all entered the same geographic area south of 58° S and between 0 and 61° W in the Atlantic sector of the Southern Ocean.

High level of synchrony The Greenland-breeding birds exhibited clear synchrony in the timing of migration, all reaching the North Atlantic stopover site, departing the wintering area, and crossing the Equator within a few days of each other, but there was no indication that they travelled together in the same flocks. Similarly, at-sea observations suggest that flock sizes of migrating terns are typically very small (<15 birds), and recent results from other migrant seabirds also indicate high levels of synchrony in timing of passage through restricted flyways, and no evidence of persistent associations between individuals (including members of a pair).


Arctic tern Northeast Greenland
An Arctic tern at Sand Island, High-Arctic Greenland.

Methods


This study used miniature archival light loggers (geo-locators) to map the migration route of the Arctic tern. The method itself is not novel and has been used successfully for more than a decade, but due to the weight of loggers, studies were limited to large-sized seabirds, such as albatrosses and shearwaters. Within the last couple of years, however, technological development has allowed these loggers to be reduced in size and weight, opening a whole new array of small to medium-sized birds to such study.


Geolocator for Arctic tern
The geo-locator weighs only 1.4 g and is light enough for the bird to carry it on a small ring around its leg.

By recording and storing ambient light intensity, the geolocators reveal information on sunrise and sunset. When these data are combined with time recordings, two daily geographical positions can be calculated, and migration routes can be mapped. Compared to conventional satellite transmitters, geolocators are much lighter and can be carried by smaller birds, and the cost is significantly lower. The disadvantage of geolocators is that they cannot transmit data, and the tagged bird must be caught again at the end of the study period – a logistic challenge that normally requires that the bird nest at the same location two years in a row – and the accuracy of a position obtained by geolocator (approx. 185 km on average), is less precise than satellite transmitter positions.


Arctic tern and geolocator
An Arctic tern equipped with a geo-locator in 2007 returns to the breeding site at Sand Island in Northeast Greenland the following year, after having completed a round-trip migration of more than 70,000 km.

In this study, we fitted miniature archival light loggers (Mk14 geolocators, mass 1.4 g; British Antarctic Survey, Cambridge), attached to plastic leg rings, to 50 breeding Arctic terns in Northeast Greenland and 20 loggers on Arctic terns in Northwest Iceland in July 2007. Logger, ring, tape, and cable tie weighed 2.0g, approximately 1.9% of mean adult body mass. The following year we returned to the study sites and were able to retrieve ten loggers in Greenland and one logger in Iceland. More birds with loggers were seen in the colony, but these could not be recaptured. A low return rate was expected as Arctic terns may shift colonies between years, or simply skip a breeding season altogether.



Study species


The Arctic tern is known to make the longest annual migration in the animal kingdom. During its breeding season, it is found far to the north where summer days are long, and it winters far south in the southern hemisphere, where the days are longest during November to February. This means that the Arctic tern probably experiences more sun light during a calendar year than any other creature on Earth. The long-distance travel of the Arctic tern is well-known both amongst researchers and in the broader public. Now, for the first time, technological advances allow us to follow the Arctic tern on its immense journey, practically from pole to pole.


The Arctic tern (Sterna paradisaea) is a medium-sized seabird with an average mass of 95-125 g and a wingspan of 75-85 cm. The species has a circumpolar breeding distribution, breeding colonially in Arctic and sub-Arctic regions of Europe, Asia, and North America, with the largest colonies found located in Greenland and Iceland. They lay one, two, or sometimes three eggs in a small scrape on the ground, making the Arctic tern particularly vulnerable to predatory mammals, including the Arctic fox.


Arctic tern breeding site
An Arctic tern at the study location, Sand Island, High-Arctic Greenland.

The Arctic tern has an opportunistic feeding strategy, with small fish and crustaceans being the most common prey items. They forage by plunge-diving and surface dipping, finding their food items in the upper layer (top 50 cm) of the water column. Like other seabirds, Arctic terns are generally long-lived, and the oldest known individual reached the ripe old age of 34 years.



Arctic tern with short legs
The Arctic tern (Sterna paradisaea) is made for a life on the wing and not on the ground. The wings are long and slender while the legs are extremely short. The Arctic tern is an opportunistic feeder on small fish and large zooplankton.

The Arctic tern is notorious for its long-distance migration between the breeding grounds and the winter quarters in the Southern Ocean. This knowledge has been obtained by ringing and observations of movements at-sea. Ringing recoveries of birds breeding in Europe and Greenland indicate that the birds migrate along the coast of Western Europe and West Africa during autumn and reach South Africa in November. From there on ringing recoveries are very scarce, but the species is observed to winter in the Southern Ocean. The migration back to the breeding ground, however, was undocumented until now.


Arctic tern at nest
An Arctic tern at its nest in West Greenland.

This study on Arctic tern migration was conducted by researchers from the Greenland Institute of Natural Resources and Icelandic Institute of Natural History, in cooperation with the British Antarctic Survey, and National Environmental Research Institute, Aarhus University. The project is adopted by the CAFF (Conservation of Arctic Flora and Fauna) Circumpolar Seabird Group as part of a larger coordinated research effort on Arctic tern migration.


Download article from PNAS on Arctic tern migration:

egevang_arte-migration_pnas.full
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