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The impact of avian flu on Western Isles seabird populations is ‘highly concerning,’ according to a new report released today (Tuesday 13 February).

Great skua, gannet and black-backed gulls are among species which have seen their numbers plummet since the disease first emerged in 2021.

The new report has, for the first time, quantified the effects of Highly Pathogenic Avian Influenza (HPAI) outbreaks on the population of seabirds.

Of the 13 species included in the study, nine showed declines, three of which are attributable to HPAI, with a further two being most likely attributable to the disease.

The RSPB said UK seabirds are in dire trouble, facing unsustainable fishing practices, predation by non-native mammals, offshore wind development, climate change and now bird flu.

Today’s RSPB Conservation Science report, ‘UK seabird colony counts in 2023 following the 2021-22 outbreak of Highly Pathogenic Avian Influenza,’ says that assessing the impact of the disease is now a top monitoring priority for seabird conservation.

The report shows that Western Isles populations of the great skua, an aggressive piratical bird familiar to all wildlife watchers, plummeted after avian flu was first noted in the species in summer 2021.

In June 2022 the National Trust for Scotland confirmed that ‘a significant proportion’ of the great skuas on St Kilda had succumbed to avian flu, following cases confirmed there the previous September.

In the report, the RSPB said: “The current H5N1 strain of HPAI has affected UK wild bird populations on an unprecedented scale … with seabirds and waterfowl particularly affected.

“The extent of reported mortalities attributed to HPAI in the UK and across Europe in 2022 demonstrated that HPAI had become one of the biggest immediate conservation threats faced by multiple seabird species, including some for which the UK population is of global importance.

“Many species impacted by HPAI are of conservation concern in the UK, and the outbreak comes on top of widespread declines reported by the latest seabird census, Seabirds Count.”

Among the sites and species monitored in the Western Isles for the report, researchers surveyed gannets on Sula Sgeir and North Rona, guillemots on the Shiant Isles and great skua in St Kilda and the Flannan Isles.

Great skua was the first seabird species in which HPAI was detected in the UK as part of the current outbreak, with the virus first recorded in Shetland, Orkney, St Kilda and the Flannan Isles in summer 2021. The report said very high mortalities were observed during 2022.

Great skua is an amber-listed species of concern, with Britain hosting 55-60% of the global breeding population and almost all of the UK population found in Scotland.

In 2023, great skua nesting sites were down by 57% on St Kilda against the baseline count data from before the virus.

That contributed to a 77% overall decline in breeding numbers in Scotland and 76% across the UK – the greatest decline of all species.

Also down across the UK, including at some sites in the Western Isles, were numbers of Arctic skua, gannet – down by 23% at Sula Sgeir – and greater and lesser black-backed gulls.

The report concludes: “Our updated colony counts generally show a highly concerning picture across our target species at sites surveyed in 2023 when compared to pre-HPAI baseline figures, with extensive declines across species and sites.

“These declines are particularly alarming given that they either come on top of previous declines experienced by some species, or have reversed trends of previously increasing populations for those few species which the census showed to be faring better.”

The pictures show a great skua in flight (Andy Hay/RSPB) and on the nest in St Kilda (St Kilda Rangers).