In my blog post of 23 June 2018, I described the occurrence of small plastic fragments in the Sooty Tern colony, which I had first noticed in the 1990s but which have been seen regularly and abundantly in the colony ever since. The origin of the irregularly-shaped pieces of remains a mystery but this year we have begun investigating how the fragments get into the colony.
The investigation comprises three parts. First, we want to identify exactly where on Bird Island the plastic fragments are to be found: are they limited to the Sooty Tern colony, or do they appear elsewhere on the island? Second, we are looking at the timescale of the appearance of plastic fragments within the areas occupied by Sooty Terns: this is to see if adults bring them in throughout the nesting season, or whether there is a specific time when the fragments make their appearance. Finally, we undertook a small trial to determine where in the sea Sooty Terns were likely to encounter plastic fragments.
To identify where plastic fragments occur on the island, we undertook transects through different habitats, marking out quadrats measuring 2 metres by 2 metres and counting the plastic pieces within. We restricted our searches to open areas of the island, as visibility on the floor of woodland was limited by vegetation (and also by the constant soil disturbance by free-roaming chickens). We found no plastic fragments on the open beach, on tracks through the staff village and hotel, or on the airstrip. As a corollary to this part of the investigation we also looked for plastic fragments close to (within 25 centimetres) nests of Brown Noddies that were nesting on the ground, and beneath trees in which a minimum of 10 pairs of Lesser Noddies were breeding – here again we found none. In the Sooty Tern colony, on the other hand, we recorded up to 40 plastic fragments per 2 x 2 metre plot. This strongly suggests that the plastic fragments are confined to the Sooty Tern nesting colony, and thus that their occurrence in the colony is related to the presence of the Sooty Terns, but not other seabird species or to human activity in other parts of the island.
The investigation of the timescale of the fragments’ appearance in the colony is in progress and we shall not know the answer until later this year. We have marked out permanent 2 x 2 metre squares in the colony and search these weekly. On each search we count and remove all fragments and categorise their colour. The first two counts revealed a lot of fragments, ranging from 1 mm to about 10 mm maximum dimension; many of these are likely to have accumulated over the years so our third collection probably represents the better starting point. These counts will continue until the chicks fledge in August-September, by which time we hope to have discovered whether the fragments arrive continuously during the 3-4 month breeding season, or whether they come in during incubation or chick-rearing. We shall also examine birds that die in the colony to see if their digestive systems reveal fragments.
The small trial to help to discover where in the sea Sooty Terns are likely to encounter plastic fragments involved placing fragments that we collected from the colony in a bowl of sea water to see whether they floated or sank. They all remained afloat for the five-week duration of the trial. Sooty Terns could thus be finding them at the sea surface and picking them up there. The birds might mistake them for food items on the surface of the water, or adults might pick up the pieces accidentally when they leave their nest sites to drink seawater close to the shore, for themselves during incubation and for their chicks after these have hatched. However, it is also possible that the prey of Sooty Terns (usually small fish and squid) ingest the plastic, which then enters the birds through their food supply.
There is still much to learn, including of course whether the plastic is having any detrimental effects on the birds. This will require a more in-depth study in future.