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.

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.

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).