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Sooty terns (Sterna fuscata) spend their lifetime at sea and return on their breeding site located at Mars Bay in thousands. More than 20’000 specimens of these birds can be observed during certain periods.

These are the sole sea birds that have managed to survive on the main island. There are other sea birds that still choose Ascension to reproduce, but only on the island of Boatswain Bird and among the cliffs or rocks that are inaccessible to cats. The terns seem to survive because they rarely stay on land and they reproduce following a strange cycle of 9 1/2 months. This means that the breeding cycle of their predators differs from theirs. In fact, they breed synchronistically in crowded colonies with millions of eggs laid at the same time. Each year, the short presence of the birds, eggs and chicks on the island produce an over-abundance of food for the predators. Hence, the latter’s population depends on the quantity of food available on the island, especially when the birds have already migrated elsewhere. This regular yet non-annual cycle probably represents the maximum rate of reproduction.

Due to the poor availability of food on the island, the Sooty terns often undergo periodical failures when attempting to reproduce. This situation results from changes and upheavals in the oceanography field brought about by events such as El Nino. In such years, many birds don't bother to breed at all and those that do, abandon their eggs or chicks later. During these periods, thousands of abandoned eggs can be found in Mars Bay. The terns can live up to many years; the bird population can therefore withstand these failures provided that they do not occur on a regular basis. The cats are to a great extent the main cause of the limitations in the development of the population. The eradication of the wild cats living in Ascension will probably allow for a rapid growth in the population of terns. However, this can only happen if the stocks of fishes are sufficient to feed the bird population.

Photos: © Fabrice Bettex / Mysterra




 
Ascension Island picture - Sooty tern

Ascension Island picture - Sooty tern

Ascension Island picture - Sooty tern

Ascension Island picture - Sooty tern

Ascension Island picture - Sooty tern

Ascension Island picture - Sooty tern
       
       
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