Aggressive and with a tendency to launch savage attacks on its feathered friends, stealing their food and sometimes eating them, has earned the great skua has earned the nickname Pirate of the Seas.

But it appears the birds are far more vulnerable than their reputation suggests, with fears growing that numbers in its traditional northern island breeding grounds have been severely affected by avian flu.

A Scottish Government report has confirmed that at least 65 dead great skuas – known as bonxie - have been found on Fair Isle, and a further 30 plus dead birds located on St Kilda since mid-July.

Birds from Fair Isle and St. Kilda have tested positive for the highly pathogenic H5N1 strain of avian influenza.

Genetic testing has shown similarities with an outbreak among a flock of approximately 14,000 gamebirds at a gamebird rearing premises in Leven in February. All of the gamebirds were culled.

It is the first time great skuas – of which 60% of the world's 16,000 breeding pairs breed in Scotland - have been confirmed to have the virus, raising concerns over how it might progress through the population. Great skuas are known to cannibalise and will chase other birds for food, raising the risk of cross-infection.

Along with concerns over the loss of high numbers of adult great skua, there are growing fears over the impact on the breeding season.

One nature watcher who explored the birds' nesting site at Hermaness on Unst, which normally sees around 1000 pairs of nesting bonxie, has reported that this breeding season appears to have produced hardly any chicks.

Writing in his blog, Mike Pennington told of his shock as he spent up to three hours exploring the breeding area at the peak of nesting season, only to find it "eerily quiet".

He added: "We weren't dive-bombed in anger once. A few half-hearted swoops, but it was as though none of them had any chicks."

Further visits later confirmed there were only a few chicks and also hardly any corpses – suggesting bonxie chicks did not grow large enough to leave identifiable remains. Mr Pennington went on to find dead skuas elsewhere on the island.

Along with the dead bonxies at the Hermaness reserve on Unst, other dead birds were found at Burrafirth, Hermaness Hill and on the Keen of Hamar nature reserve on the island.

The disastrous breeding season comes after last year produced just seven chicks and 21 corpses at the Hermaness reserve.

Fair Isle's great skua colony is also said to have suffered a very poor breeding season: of the 535 pairs present, the number of fledged chicks only just numbered into double figures.

As well as the H5N1 strain, a different strain of bird flu, H5N8, emerged at a poultry premises in Sanday, Orkney, in December last year. Since then, the Scottish Government's surveillance programme has identified avian influenza H5N8 in dead wild birds in Cupar, the Dornoch Firth, Fraserburgh, Clackmannanshire, Montrose, South Uist, Ellon and North Berwick.

The risk to human health from H5N8 is said to be 'low' and from the H5N1 virus strains 'very low', while food standards bodies say avian influenza poses a 'very low' food safety risk and does not affect the consumption of poultry products, including eggs.

However, there are concerns that the outbreak among great skuas could have deeper implications for other wildbirds, due to their predatory and aggressive nature.

Great skuas are one of Shetland's iconic birds, arriving from north-west Africa in April and remaining until September. They are roughly the size of a herring gull and are known for their fierce, fearless nature.

Shetland-raised chicks have gone on to be found in locations ranging from Algeria, Guyana and Morocco to Malta, France and Greenland.

They typically feed on discards from fishing boats and, like birds of prey, attack and kill other birds such as fulmars and puffins, and rabbits. However, they also steal food from other seabirds, raising the risk that an infected skua may come into direct contact with another bird which then escapes but is left carrying the infection.

According to the Scottish Government report, it's possible the bonxie were infected by predating, scavenging, or eating diseased seabirds and waterfowl including ducks, geese, swans and greylag geese, or from bathing in lagoons used by infected waterfowl.

The risk for wild birds within the immediate area of the colonies at Fair Isle, Unst and St. Kilda, is said to be 'high'. However, the report says there is only 'low risk' to domestic poultry.

It adds: "Infected birds may not be capable of flying long distances, hence helping to contain direct transmission of disease to other wild birds."

A spokesperson for bird charity, the RSPB, said: "This predatory seabird species has a small and restricted global breeding range, with a world population of around 16,000 breeding pairs. At the last census in the year 2000, 60% of the world breeding population was found in Scotland, with over three-quarters of Scottish birds nesting in Shetland and Orkney.

"In recent weeks, sick and dying birds have been found at multiple locations in Shetland, and breeding success has been unusually low at some key Orkney and Shetland breeding colonies.

"It is unclear at this stage how the birds contracted the virus, or what impact this Avian Influenza outbreak might have on our globally important breeding population of Bonxies.

"RSPB Scotland calls for the urgent adoption of policies and conservation actions that will maximise the resilience of Scotland's seabird populations in the face of multiple threats, including climate change, bycatch mortality, the impacts of non-native predators and disease outbreaks."

Jeff Waddell, Senior Natural Heritage Advisor for the National Trust for Scotland, which owns Fair Isle and St. Kilda and is responsible for land on Unst, said: "Due to the unusually high mortality among St Kilda's Great Skuas on St Kilda, we forwarded ten carcasses for testing and confirmation if this is also a bird flu outbreak.

"We have just received a report confirming that one of the carcasses has tested positive for H5N1. Thus far we have observed 50 dead great skuas.

"The UK Government's Animal and Plant Health Agency (APHA) has published additional biosecurity guidance and a RRA (rapid risk assessment) for Fair Isle.

"Although APHA note that H5N1 bird flu does not affect humans, and this outbreak in Great Skuas doesn't add to the general background risk that H5N1 will transmit to poultry either, all possible precautions are being taken as per the guidance and these will be applied in Unst as they have been on Fair Isle, and implemented on St Kilda too as a precaution.

"Visitor numbers to NTS land on Unst and to St Kilda are relatively low and we anticipate that the impact of the virus should be minimal."

Government advice is that, as a precaution, people should not touch dead or sick birds. If the public finds dead birds, they should be reported to a Defra helpline on 03459 33 55 77.

A Scottish Government spokesperson said: "We are aware of a positive finding of H5N1 in one great skua on Fair Isle, Shetland Islands, in one on the Flannan Islands, Outer Hebrides, and in two in St Kilda, Outer Hebrides. There is no suggestion that the four cases identified have led to mass casualties in the great skua population in Scotland. Due to risk assessments carried out, we are not currently planning to introduce control or monitoring zones. However, we will continue to monitor the situation and we would encourage anyone who finds five or more dead birds of the same species in the same location and at the same time, to report these incidents."