Walking north from the hotel along Bird Island’s west beach during the Sooty Tern breeding season, one cannot escape from the birds’ presence. In addition to the incessant calls emanating from the nesting area, the sandy beach reveals a constant moving network of shadows, cast by over-flying birds as they criss-cross the beach. Some of the to-ing and fro-ing involves birds arriving at and departing from the colony to feed, but is produced mainly by birds taking a short trip to the sea just beyond the breakers to drink. Sooty Terns, like most of Seychelles’ seabirds, drink sea water and during the hotter parts of the day their thirst needs more frequent quenching. While we would be unable to survive such salt intake, the birds have a large gland, just above the eye, that excretes the excess salt that the birds have ingested. The salt drips from the nostrils, sometimes leaving a white deposit on the otherwise black bill.
For a bird that spends so much time standing or sitting in strong tropical sunshine, it seems strange that it has evolved to have black plumage on its back and the top of the head. We would expect this to absorb
heat and various behaviours are used to counteract potential heat stress. Sooty Terns that are incubating, or standing over the egg to shade it from the sun, or merely standing in the colony or on the beach, face into the wind. They often droop their wings, apparently creating a wind tunnel beneath their bodies. This exposes the undersides of their bodies to the wind and when birds are caring for the egg this shades the egg from the direct sunlight, possibly preventing overheating in the heat of the day. They also raise their back feathers, presumably increasing the heat insulation offered by these feathers. Additionally, birds can often be seen panting with the bill open.
But the most conspicuous aspect of cooling behaviour to a beach walker is the short flights to and from the sea. In addition to drinking by dipping that bills into the sea surface, incubating Sooty Terns often wet their feet and bellies but they do not settle on the water, remaining in flight throughout. The adults might derive some cooling benefit from dipping belly and feet in the water but this might be more involved with maintaining the appropriate temperature and humidity for the egg.
The shadow-producing flights over the beach are more frequent in the hotter parts of the day and are all part of the free entertainment provided by Bird Island’s spectacular Sooty Terns!
Wild Bird Conservation 