Bird Island Sooty Tern update

Eggs cover the ground! (Chris Feare)
Eggs cover the ground! (Chris Feare)

The main part of the colony is now packed with birds nesting at high densities of up to six nests per square metre. A remarkable sight and the volume of noise is astounding. More birds are continuing to arrive, however, and occupying other parts of the open plain at the north of Bird Island.

About 11 hectares of ground are kept clear for the nesting Sooty Terns but prior to the birds’ arrival further ground preparation is necessary. As far as possible the area is mown in April and May to keep the vegetation short. Island conservation staff additionally trim some of the denser bushes to allow birds to nest underneath, and when possible they also try to remove the entwining vine Lalyann san fan (see post of 18 June) to prevent birds becoming entangled. The success of this management depends on rainfall; in 2016 the early onset of the south-east trade winds, which generally (but not always) provide drier weather did provide good conditions and the birds started laying earlier than they have done for several years. Coupled with this year’s dry weather, accompanied by the desiccating south-east trade winds, colony management has produced perfect conditions for Sooty Terns.

On top of this 2016 appears to be a bumper year for them. In most years some parts of the northern open plain remain unoccupied but in 2016 virtually all parts have nesting birds. In addition they have occupied low vegetation along the crest of the western beach extending south as far as the front of the hotel.

Sooty Terns that have been incubating for about three weeks - note that there are few birds in the air (Chris Feare)
Sooty Terns that have been incubating for about three weeks – note that there are few birds in the air (Chris Feare)

Differences between areas that were settled early and those that are being occupied now are readily apparent. Few birds are in the air over areas that Sooty Terns occupied three weeks ago and the birds there are (relatively) quiet. Where birds are arriving now, however, the air is full of wheeling Sooty Terns and their raucous calls while those on the ground are calling also as they jostle for a place to lay their eggs. Most of the birds that are circling these areas now are in pairs and have very long outer tail feathers, signifying that they have arrived recently and have not spent time displaying and establishing nest territories on the ground; during these activities the outer tail feathers become worn and can even break off.

A part of the colony where birds are still arriving to establish nest sites - the air is full of birds and noise (Chris Feare)
A part of the colony where birds are still arriving to establish nest sites – the air is full of birds and noise (Chris Feare)

Our observations are showing that at present nesting Sooty Terns are finding plenty of food within easy reach of Bird Island this year so that foraging birds can return reasonably quickly to relieve their incubating partners on the nest. However, the fish and squid taken by the birds can sometimes disappear, leading to sudden periods of food shortage. This happened during the last two years so we hope they experience better times this year.

At present we lack information from other Seychelles Sooty Tern colonies and so we do not know whether the food supplies in the proximity of Bird Island are encouraging birds from elsewhere to seek nest sited here. Could this account for the unusually large numbers this year?  Or could events elsewhere, such as the cyclone Fantala that stuck Farquhar Atoll on 17 April, have forced birds from other colonies to move to Bird Island? Another mystery surrounding the lives of Sooty Terns!

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