I have posted before about some of the incredible migration flights undertaken by birds, such as the nonstop flights of Bar-tailed Godwits across the Pacific or the circumoceanic wanderings of Sooty Shearwaters. Red Knots travel each year from Patagonia to the Arctic and back, as do some other shorebird species. Another bird species that undertakes impressive migration flights is the Arctic Tern (Sterna paradisaea). Every year, Arctic Terns make a 43,000-mile round trip between their wintering grounds in Antarctica and their breeding grounds in the Arctic.
The terns' routes were finally mapped through a banding study that outfitted each of the banded terns with tiny geolocators to track the birds' geographical locations along the way.
The first surprise is that the terns do not make straight for the Antarctic when they leave the Arctic, but make a lengthy stop-over in the middle of the North Atlantic, about 1,000km (620 miles) north of the Azores.Terns take one of two routes on the way south:
Here, they feed on zooplankton and fish to fuel themselves for the long journey ahead.
The birds then head south along the coast of western Europe and western Africa before making a choice, either to continue hugging Africa or sweep across the Atlantic from the Cape Verde Islands to continue the journey along the Brazilian coast.
About half the birds that were tracked decided to take the South American path. It is not clear why, but the researchers believe wind might make either route seem favourable to the terns.
After spending their northern winter months in Antarctic waters, the terns then fly back towards the Arctic.
But rather than retracing their southward flight paths, the birds follow a gigantic "S" pattern up the middle of the Atlantic Ocean.
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. Credit: Greenland Institute of Natural Resources.
The spring route carries the terns northward along the west coast of Africa and then along the east coast of North America. (See the maps included in the BBC article.) This may mean traveling a longer distance than a more direct route would be, but the article's authors explain the different migration routes in fall and spring based on prevailing winds. It may be more energy efficient for the birds to move clockwise in the North Atlantic a counter-clockwise in the South Atlantic since they can benefit from a tailwind both ways. It is worth noting that ocean currents follow a similar pattern and could potentially influence the ease of finding food along the way.
Whatever the reason for their routes, these terns give us an additional reason to marvel at the natural world and its avian inhabitants.
Update: I added the map of the terns' routes from the project website, which has additional images and information: http://www.arctictern.info/