British High Commission sponsors new Sooty Tern research in Seychelles |22 September 2014

The Sooty Tern or Golet is the provider of the ‘birds’ eggs’ that create so much excitement for many Seychellois in June and July each year.

Chris Feare, an ornithologist from the United Kingdom, has been undertaking research on them since 1972, building up a picture of their life histories with the long term goal of estimating how many eggs can be taken safely each year without harming the Sooty Tern populations of Seychelles.

This is a highly complex estimation that requires detailed information on the birds’ production of eggs and chicks, and the survival of young birds up to the time they first breed and then their survival as breeding adults.

These are exciting times, however, with the development of new technologies that for the first time allow the tracking of individual birds while they are away from the breeding colonies. From 2011 to 2013 Chris and his Seychellois friend Christine Larose fitted tiny geolocators to rings on the legs of some Sooty Terns on Bird Island. Recaptures of these birds revealed that they had migrated over large distances across the Indian Ocean, ranging as far as the Bay of Bengal in the north to the southern tip of Madagascar in the south, the northern Mozambique Channel in the west and the coast of Western Australia in the east.

In 2014, Chris and Christine wanted to investigate where Bird Island’s Sooty Terns feed while they are incubating their eggs. This is a critical time for them as the birds are limited in how far they can travel to search for food, mainly small fish and squid that are driven to the surface of the sea by tuna, as the Sooty Terns must return regularly to the island to relieve their partners if the eggs are to hatch successfully.

British High Commissioner Lindsay Skoll took a keen interest in the proposed study at an early stage of its development such that the British High Commission became the main sponsor of the project. This involved the fitting of tiny GPS loggers to Sooty Terns while they were incubating their eggs in June-July, waiting for them to depart on a foraging trip and then catching them again on their return in order to download the location data, accurate to about three metres that had been stored by the loggers.

The first loggers were attached to birds in late June, when food was plentiful, and foraging trips lasted one to two days. These birds fed over the shallower waters on the Seychelles bank, spending most of their feeding time south of Bird, in the vicinity of Aride and Silhouette. Their round trips involved journeys of 100 to 150 kilometres. As the study progressed, however, foraging trips became longer, taking six to 13 days. This and other observations indicated that Sooty Terns were finding it harder and harder to find food and many birds abandoned their eggs. In the first half of July two birds were tracked over six-day foraging trips and these made remarkable journeys in search of food. One headed west from Bird Island and fed over 900 kilometres away, half way to the African coast, making a round trip of over 2,000 kilometres. The other bird headed north from Bird Island and after flying about 450 kilometres turned north-east, flying for about another 350 kilometres before finding food and spending 36 hours in this location. It then flew south for 560 kilometres and then decided to retrace its steps, finding more food before returning the remaining 320 kilometres to Bird Island and completing a round trip of about 2,000 kilometres.

We do not know what caused the food shortage in July but the data show how sensitive Sooty Terns are to sudden changes in their food supply, the lengths they will go to in search of food while attempting to breed, and also the impact that food shortage has on incubation behaviour and the birds’ success in rearing young. It is hoped that this year’s study will form the basis of a wider investigation of the relationship between Sooty Terns and oceanic conditions in collaboration with other ornithologists in the Indian Ocean and in other ocean systems.

Chris and Christine wish to acknowledge invaluable support from others who made the 2014 study possible, namely James Cadbury and the Percy Sladen Memorial Fund for financial help, Bird Island and its owners for their enthusiastic contribution of facilities, PathTrack (UK) for advice on their GPS loggers throughout the fieldwork and the Seychelles Bureau of Standards for their approval of the project.