There can be few who are unaware of the ubiquitous problems faced by the world’s oceans as more and more tonnes of plastic contaminants enter them. A wide variety of plastic bottles, sheeting, fishing waste (buoys, line, fish aggregating devices [see my blog of 13 June 2017]) polystyrene packaging, flip-flops and other shoes, wash up regularly on tiny and remote Bird Island’s tide lines. Closer to major shipping routes, Seychelles’ southern islands and atolls, Farquhar, Aldabra, Cosmoledo, Astove and Assumption, experience much more serious bombardment of their shores and on one occasion, under very calm conditions, off Assumption I saw the sea surface to be littered by plastic from the island shore to the horizon.
In the 1990s I became aware of small fragments of plastic on the ground inside the Sooty Tern colony on Bird Island. These fragments are readily visible as we walk through the colony, irregularly shaped, often four-sided but sometimes circular, and rarely more than 10 mm maximum dimension. I still have no idea how they get there! But they do seem to be related to the Sooty Terns’ presence.
This year I have looked a bit more closely but have not made a detailed study. The first general point is that they are much more abundant within the Sooty Tern colony than elsewhere. We do see them in much smaller numbers on sections of the beach where some flocks of Sooty Terns roost at night but are absent during the day, but elsewhere on stable open sandy areas we have not found them. Nor do we find them in other parts of the island, for example on the airstrip or around the hotel or staff accommodation – these are areas of short grass where plastic fragments would be visible but their absence here, along with the absence of Sooty Terns, suggests that the distribution of plastic fragments is associated with the presence of these birds. The much greater abundance within the Sooty Terns’ traditional nesting area could represent an accumulation over many years, along with an annual input from breeding birds.
Within the nesting colony most fragments are seen within about 10 cm of nest depressions and occasionally within them, very close to the egg. In the 1990s I did place plastic fragments near nests to see if incubating Sooty Terns pulled them closer, but they did not, suggesting that they are not used as some kind of marker or nest decoration.
Another notable feature is the colour of the plastic fragments. They are almost entirely blue, turquoise or green. This year we have noted a few pink or red ones but these are exceptional. While the plastics washed up on the beach appear in a wide range of colours, we have never found fragments in the colony that are black, white, grey, yellow or translucent. If Sooty Terns are indeed the importers of the plastic fragments, the colours of the fragments found in the colony suggest that some selectivity is involved, favouring blues and greens. In other species of seabird it has been suggested that feeding individuals might mistake pieces of plastic for fish, but we are unaware of support for this possibility in Sooty Terns.
For other seabird species, and other marine life, ingestion of plastic has been found to be highly injurious, sometimes fatal. We do not yet know whether Sooty Terns are ingesting the plastic, and then regurgitating or excreting it in the colony, or if it is regurgitated in food given to their chicks. Thankfully, we have not seen signs of unusual mortality in adult Sooty Terns on Bird Island or of young birds in the colony.The presence of these fragments in the colony, however, is another sign of the ubiquity of plastic in the oceanic environment and of a potential man-made risk to one of the most numerous tropical seabirds.