A Tern on the Tide

Common Tern (Sterna hirundo), non-breeding plumage.

On a warm and breezy December morning, Jennifer McKenzie of the Hermanus Animal Hospital was walking along the Vermont coastal path when her eye was caught by the wing of a bird lying on the tideline among the pebbles and shells. Closer investigation revealed the sun-bleached corpse of a small tern, with a small aluminium ring attached around one of its tiny legs.

Jennifer contacted me and once I had retrieved the dead bird – and tentatively identified it as a Common Tern (Sterna hirundo) – I was immediately in touch with the South African bird ringing authority (SAFRING) who have an online record submission facility. The data form was duly completed and I awaited feedback on the origin and age of the tern. Since the ring had the words ‘Museum Brussels’ embossed above the number, it had evidently been ringed in that northern European country, but how old was this small migratory bird that had flown a distance of over 10,000 km to reach our shores?

Months passed before I received feedback. Yes, it was Sterna hirundo and had been trapped and ringed (as a first year/immature) in Belgium, at Heist, to be precise. Then, what I had been most interested in, the date of capture: 24 August 2008. Nine years, three months and 14 days before it turned up dead in Hermanus.

Now I got my calculator out, and launched Google Earth on my desktop. Although the distance between Heist and Hermanus is 9,690 km, migratory terns – being coastal seabirds – do not fly in a straight line over the African continent, but must follow the coastline, or perhaps use trade winds further offshore. This means that the journey south is somewhere in the region of 13,000 km and, presuming that these terns return home to breed by the same Atlantic route then they must cover at least 26,000 km each year. If we imagine that this little bird stayed put in Europe for its first year, and only undertook migration as an adult, then it would have completed the return trip eight times, with one final journey south in the European autumn of 2017. Add that up, and you have a distance of 221,000 km . . . a bundle of muscles and feathers weighing about 120 grams travelling a distance equivalent to more than halfway to the moon. Rather incredible.

More amazing, is that this really was still a young bird. The oldest Common Tern listed on the EURING (the European bird ringing association) database was 33 years of age! 

Duncan Butchart, Hermanus