Tracking of Arctic terns Sterna paradisaea reveals longest animal migration

Arctic terns breeding in Greenland and Iceland were fitted with miniature light-level geolocators, which were recovered when the birds returned the following season, and their annual routes reconstructed.

Studied in

Sterna paradisaea

Sample size

Arctic terns carrying light-level geolocators, recovered the following year

Birds followed a winding route with an extended stopover in the North Atlantic and a southbound track down either the African or the South American side, covering annual distances far greater than the straight-line pole-to-pole distance.

• Geolocators must be recovered, so the sample is restricted to birds that survived and returned to the same colony.
• Light-level geolocation gives positions accurate to roughly a hundred kilometres, and fails near the equinoxes.
• Two colonies; other populations may travel differently.

• Waiting a year to find out where the bird went

  How far does an Arctic tern actually travel in a year — measured, rather than estimated?

• Bird migration

  Arctic terns travel further in a year than any known animal — but not in a straight line. The tracked route wanders, pauses in the mid-Atlantic for weeks, and is far longer than the gap between its ends.

• How we know where animals go

  A geolocator stores light levels and transmits nothing. To learn anything you have to catch the same bird again the following year — so every route measured this way comes from a bird that survived.