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A powerful bird known as the pirate of the seas has declined dramatically because of avian flu, the RSPB says.

Great skuas soar around the UK's coasts stealing other birds' food but their numbers in 2023 were down by 76%, the charity says in a report.

Populations of gannets and roseate terns were also seriously reduced after avian flu killed thousands of wild birds in 2021-22.

The numbers of the three species had been rising before the outbreak.

The H5N1 strain of avian flu spread to wild birds in summer 2021, causing thousands of creatures to die.

The findings make it clear that avian flu is "one of the biggest immediate conservation threats faced by multiple sea-birds", says the RSPB.

"This is a wake-up call as to how serious avian flu is and it's coming on top of multiple other threats that these species face," says Jean Duggan, RSPB avian influenza policy assistant.

The RSPB surveyed 13 bird species in May-July 2023, and concluded that avian flu had caused the decline of great skuas, gannets and roseate terns, and was very likely to have caused reductions of sandwich and common terns.

Gannet populations are down by 25%, roseate terns by 21%, sandwich terns by 35% and common terns by 42% compared with a major census of bird populations in 2015-2021.

Almost the entire UK population of great skuas live in Scotland. In 2022, at least 2,591 great skuas died - 1,400 from one colony alone on Foula island, Shetland.

The total number in the UK has declined from 9,088 to 2,160.

Ms Duggan highlighted that Britain was pivotal in protecting the bird worldwide because so many breed in Britain.

"It's empowering to realise that if we take the right actions in the UK it will actually benefit global populations to a very significant degree," she says.

Gannets were also badly affected in 2022, with 11,175 killed in Scotland and an estimated 5,000 mortalities at Grassholm in Wales.

In 2023 the total number counted in the UK had declined by 25%, going from 227,129 to 171,048.

Avian flu has become less acute in the UK in recent months, but it has caused mass mortalities of birds in other parts of the world.

In January it was detected in elephant and fur seals in Antarctica for the first time.

"While the virus is still present globally, UK birds are still at risk, and the virus will continue to mutate. We need to consider it a long-term threat," says Ms Duggan.

Climate change, mortality linked to fishing, the effects of offshore wind developments and a reduction in the availability of food are other threats facing UK sea-birds, according to the RSPB.