It is a tiny spit of land that lies in the Firth of Clyde two miles off the coast of Troon where colonies of seabirds have made their homes for centuries.

Now scientists have discovered that the majority of nests built by the birds have been constructed using discarded plastic debris.

Almost four in 10 of the nests surveyed on Lady Isle, said to be Britain’s first seabird reserve and designated an area of special protection by Scottish Natural Heritage, contained plastic, with the amount varying between species.

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More than a third of herring gull nests surveyed contained plastic, while for the great black-backed gulls it was just over half and for European shags almost all at 80%.

Researchers from the University of Glasgow believe the birds’ individual nest-building behaviours are behind the difference with shags known to reuse and rebuild their nests every year, allowing the plastic to build up over time.

The survey, carried out in 2018, found that the plastic has been washed ashore, where it was collected by the birds and used to build their dwellings.

Researchers said it is important to gather more information about the impact of plastic in nests as it could affect the quality of the structure with detrimental effects for the eggs and chicks, while there is also a risk birds could become tangled up in it.

Plastic in nests has been identified as being mostly from consumer waste thrown away in built-up areas.

Dr Ruedi Nager, a seabird ecologist and senior lecturer at the University of Glasgow, said: “They end up in seabird nests, not because seabirds actively pick them up in built-up areas and carry them to their nest, but because are brought there passively by marine currents.”

During the survey researchers examined 1,597 nests on the island off the Ayrshire coast and found that 625 contained some plastic.

Plastic was found in around a quarter of both cormorants’ and lesser black-backed gulls nests.

Danni Thompson, a researcher volunteering with Dr Nager, looked more closely at the herring gull, the most abundant species nesting on Lady Isle.

She said: “As herring gulls often forage in landfills, we wanted to see if they were swallowing plastic whilst eating and then bringing it back to the nest.”

The research suggests that the plastic in nests arrived by means other than from debris that birds ingest while foraging in populated areas.

From photographs of the nests and plastic found in pellets of regurgitated food remains at the nest site, researchers were able to compare types and colour of ingested plastic and plastic incorporated in the nest.

If the likely source of plastic in nests was from plastic debris that birds ingest while foraging in populated areas, then the researchers would have expected a high similarity between plastic debris in pellets and nests,

“But the plastic types in their diet were different from those found in the nest, which tells us that the plastic in nests arrived by different means,” Said Dr Nager.

All of the nests on Lady Isle were mapped and tested whether those containing plastic were equally distributed across the island.

Results showed nests on the north of the island, which are closer to the outgoing tide from the mainland, were more likely to contain plastic.

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Scientists took this to suggest the plastic in the nests originated from the mainland and was washed up on the shore.

They said further research is needed to identify whether or not plastic presence in nests has an impact on seabird populations, which are facing a global decline in part due to plastic pollution through ingestion, fatal entanglement and nest incorporation.

It is thought that plastic debris in the nests could potentially affect the quality and properties of the nest with detrimental effects for the eggs and chicks.

Monitoring plastic in nests using photographs to assess the type and quantity can allow scientists to monitor changes over time as well as tell them where the plastic came from.

Identifying the potential sources of plastic could also help conservationists to plan future management action, such as targeted beach cleans, to reduce the negative impact of pollution on Scotland’s struggling seabird population.