The longest commute in nature
AFew animals travel as far as the Arctic tern, a slender seabird weighing little more than 100 grams. It breeds during the northern summer on coasts around the Arctic, then flies south to spend the southern summer near the edge of the Antarctic pack ice. Because it moves between two polar summers, it probably sees more daylight each year than any other animal.
BFor a long time the length of this journey was estimated from sightings at sea and from birds that had been ringed at their colonies. The picture changed when researchers began fitting terns with geolocators, tiny devices weighing about as much as a paper clip that record light levels. From the times of sunrise and sunset, scientists can calculate a bird's approximate position each day. One study of terns from Greenland and Iceland found that the average round trip was around 70,000 kilometres.
CThe tracks also showed that the birds do not take the shortest route. On the way south, most paused for several weeks in an area of the North Atlantic where cold and warm water meet and food is plentiful. They then followed either the coast of Africa or the coast of South America. On the return flight, they travelled along a huge S-shaped path through the middle of the Atlantic. This longer route appears to save energy, because it allows the birds to take advantage of the prevailing winds.
DFeeding on the journey is essential. Terns rarely settle on the sea, so they feed on the wing, catching small fish and krill near the surface in brief dives. Since many individuals live for more than thirty years, a single bird may cover a distance in its lifetime roughly equal to three trips to the Moon and back.
EResearchers are now asking how a warming climate will affect this pattern. If the feeding area in the North Atlantic becomes less productive, terns may need to stay there longer or search for new feeding grounds, adding to an already demanding journey.