Where do Bird Island Sooty Terns spend their holidays?

Bird Islandés Sooty Tern colony is the best studied in the world, thanks to my own good fortune in having the opportunity to study them over a long period, and to the island’s owners for supporting my research for almost 40 years. Much of what we know about Sooty Terns and their behaviour has been discovered on Bird Island but a major mystery remains – where do they go when they leave the island at the end of each breeding season? In 2011 we set the wheels in motion to discover their off-season wanderings.

Recently developed electronic devices are now small and light enough to attach to Sooty Terns. These ‘geolocators’ record data from which location can be determined and, once the bird is recaptured, these data can be downloaded on to a computer programmed to map the movements of the bird during the interval between marking and recapture.

As luck would have it, our only partially successful fundraising almost came to nought as 2011 was one of the latest Sooty Tern breeding seasons on record over much of the Western Indian Ocean, from Seychelles down to Madagascar. In April and May there had been a die-off of Sooty Terns in Sri Lanka, including one of the birds I had ringed in Seychelles, but we do not know if the late arrival for breeding was in any way connected with this. Throughout our three weeks on Bird Island, which should have coincided with peak laying, few birds landed in the colony and those that did showed very little interest in courtship or territory acquisition and few eggs were laid. Moreover, many of the eggs that were laid were deserted a few days after laying. After we left towards the end of June, however, Marie France Savy reported that laying began in earnest.

Our team, comprising Ron Summers (a former colleague who is now tracking shorebirds using geolocators), his wife Bozena (herself a shorebird expert and also tour leader), Christine Larose (a Seychelloise with a love of islands and their natural history) and me, eventually managed to deploy our 60 geolocators on incubating adults although some of these deserted their eggs after a few days, just as their untagged neighbours were doing.

By now those birds that have succeeded in rearing chicks this year will be leaving the colony with their young while those that failed to nest may already be far out in the Indian Ocean. Where they feed during this time could provide information on oceanic areas worthy of protection, some of which could be important for other species, including fish and marine mammals as well as birds.

So how do we discover where they have been? Putting the geolocators on this year was the easy part. Next year we have to find the marked birds in the Bird Island colony, catch them and download the data from the geolocators. Finding the marked birds is the problem. We are allocating a month to walking through the colony, now numbering about 300,000 pairs, while the birds are incubating. This is when they stay close to their nests even at close human approach and we can examine their legs for rings and geolocators when they stand up. This is not a job for the faint-hearted! It requires great perseverance and concentration in the shadeless open colony and, especially as hatching approaches, under constant attack from the adult Sooty Terns who peck viciously at ankles and sometimes at our heads as we walk slowly among them. We have to wear hearing protection against the incredible volume of sound produced by the birds and we have to weather the constant rain of droppings! Once seen, marked birds are fairly easy to catch using small hand nets. When we find our marked birds all the effort will be worth-while – and be sure the results will find their way on to the Bird Island web page!

We are grateful to Bird Island its continuing support for Sooty Tern research in its amazing colony, and especially to Marie France who ensured all our needs were met throughout our stay at a time when the island was closed – a great honour! We also thank our sponsors for their generosity in supporting this project: the Percy Sladen Memorial Fund, Dr James Cadbury, Robert Gaines-Cooper, Brian and Margaret Jasper, Dr Matthieu LeCorre through a Pew Marine Conservation Fellowship, Dr Kang Nee, Amanda O’Keefe and Colin and Fiona Short.

We need further funding to support the search for the marked birds in June 2012. If any of Bird Island’s visitors would like to support the project we should be grateful for any contributions. For further information please contact both me and Ron Summers for further information (we both work away from home for variable periods so please send emails to both of us to ensure an early reply).

July 13, 2011 12:00 am

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