Return of an old friend

Christine with her “trophy” – a Sooty Tern wearing the geolocator that was fitted seven years previously (Photo: Chris Feare)

Part of our work on Bird Island involves walking slowly through the Sooty Tern colony looking for birds that we have ringed in the past, some as long ago as 1972. It is not an easy task. It involves long periods of intense concentration and patience while being subjected to attacks to our heads and legs, and the frequent sensation of warm droppings landing on various parts of our bodies! And of course we have to tread exceedingly carefully to avoid stepping on eggs – in the Bird Island colony there are up to seven nests per square metre.

During our ring search on 12 June Christine signalled that she had caught something out of the ordinary that she wanted me to look at. It proved to be a bird that we had ringed in 2011, seven years ago. But in addition to the ring on one leg, we had attached to its other leg a tiny device called a geolocator. Geolocators have the capacity to record light levels and time, along with seawater sensors. The data are stored on the device and the marked bird must be recaptured, when it returns to the breeding colony the following year, to retrieve the information. From these data it is possible to estimate the approximate (accurate to within about 180 km) geographic location of the bird, and thus to track its movements between release and recapture.

A geolocator attached to a plastic ring on the leg of a Sooty Tern in 2011 (Photo: Chris Feare)

The data from this study have already been published ( and were outlined in my blog of 7 December 2017. The paper describes, for the first time, the dispersal and migrations of Bird Island’s Sooty Terns.

The tiny batteries that power geolocators only last about two years and it is unlikely that the one Christine has just found will provide useful information. But it is good to know that this Sooty Tern was able to carry the device for seven years and return to breed on Bird Island with no apparent ill effects – its body weight was within the range of other birds weighed this year, it showed no signs of discomfort or wear on the leg carrying the device, and it was incubating its egg after establishing a nest territory in one of the densest parts of the colony.

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