August is the month when the strength of the south-east trade winds reaches its maximum in Seychelles, leading to rough seas and large swells. The trade wind season has always been regarded as the dry season. However, changes do seem to be afoot and recent years have seen more rainfall, sometimes heavy. The big swells crashing on to Bird Island’s beaches erode and re-distribute the sand. If the erosion reaches the margins of the Sooty Tern colony this can be catastrophic, leading to the death of thousands of chicks (see blog of 27 July 2017).
While Sooty Terns breed annually in Seychelles, on other islands they sometimes have different schedules. For example, on Ascension Island in the Atlantic they breed at approximately 9-month intervals, while on some other islands they have been recorded nesting at 6-monthly intervals. In this case, however, adults that successfully rear a chick most likely return 12 months after their breeding attempt to commence the next.
In Seychelles, the Sooty Tern’s smaller cousin, the Bridled Tern, nests at 8-9 month intervals. Elsewhere, for example on the southern Arabian coasts and in Western Australia, is strictly annual in its breeding. Thus this relative also shows adaptability in its breeding schedules, but for neither species do we understand the biological or environmental bases of the different schedules in different colonies.
Bridled Terns in Seychelles nest mainly on small rocky islands that are free of introduced predators. They do not nest on Bird Island (although in the 1990s one individual laid an egg on a log at the top of the beach) but non-breeding birds roost commonly in tall Casuarina trees along the west coast of the island and undertake their moult while on the island. This is the case at present: a few hundred Bridled Terns are roosting each evening, flying up and down the coastal strip of Casuarinas giving their plaintive calls before settling in the trees. During our annual visits to Seychelles we sometimes see Bridled Terns but on others we do not, depending on their breeding status on other other islads in the archipelago.
August is also the time when migrant birds from the north begin to arrive. Diminutive Saunders’ Terns and large Crab Plovers, with their long blue legs and massive black bills, are arriving from breeding grounds in the northern Indian Ocean, and migrant shorebirds from much further north, often as far as the arctic, are also beginning to appear. One of the first such arrivals this year is a Grey Plover which still retains some of its breeding plumage. This bird is also wearing leg rings, a metal one on one leg and a larger white plastic ring on the other. Frustratingly, it is very wary and so far has not allowed sufficiently close approach to see if the plastic ring carries letters or numbers, which might help us to trace its origin.
Our Great Cormorant, which arrived on Bird Island in April 2018 (see blog of 14 June 2018) and appears to have taken up permanent residence, continues to entertain. It seems to do most of its fishing in the shallow lagoon inside the coral reef on the east coast. Once replete, it rests, preens and dries its wings on the bow of a small fishing boat anchored in the lagoon. In the evening, however, it flies to the west coast to roost overnight high in a Casuarina tree. During our residence on the island in June and July this year it arrived at its roosting tree with incredible regularity each evening – at 1825, irrespective of cloud or sunshine. It now appears to have modified its schedule, however: on our first evening here on 20 August, it flew into its favoured tree at 1810. Perhaps it has reset its watch for local winter time!