Virus may have impacted on birds of prey numbers

​Eagles may have been particularly affected by bird flu.​Eagles may have been particularly affected by bird flu.
​Eagles may have been particularly affected by bird flu.
The breeding success of a number or bird of prey species, particularly eagles, may have been impacted by avian flu, according to a report published by NatureScot.

The widespread effects of the virus on seabirds and wildfowl in Scotland and throughout the UK have been well documented, but the effect on birds of prey is less well known. A new analysis by the British Trust for Ornithology (BTO), using data from the Scottish Raptor Monitoring Scheme, found that the proportion of golden eagle pairs successfully rearing young declined from 48% in 2021 to 28%.

During the same period, breeding success for white-tailed eagles dropped from 67% to 45%.

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White-tailed eagles appear to have been most severely impacted in coastal areas, suggesting a possible link between infected seabirds and waterfowl that eagles may have subsequently preyed on or scavenged. The largest declines recorded for both eagle species were in Lewis and Harris, where breeding success of golden eagles fell from 55% to 16%, and breeding success of white-tailed eagles declined from 66% to 24%.

The first cases of avian flu in birds of prey were detected in November 2021, and by April 2022, there were positive tests results for a range of species. The study found evidence for regional impacts on some species, such as lower breeding success for ravens in Orkney and Shetland, and for peregrines in Tayside. However, these impacts appear to be more localised than for eagles.

Other factors such as weather and prey availability can affect breeding success in birds of prey, leading to lower brood sizes or fewer chicks fledging, but the 2022 data showed that complete nest failures were more frequent than partial successes. This is consistent with the effect of a highly contagious disease such as avian flu.

John Allan, NatureScot ornithology adviser, said: “Scotland still has strong eagle populations but these findings are very concerning. So far we have had fewer positive test results among birds of prey this year than last year, but it is early in the season and we can’t be complacent. All partners in Scotland’s Avian Flu Task Force are continuing to work hard to understand this virus to make our wild bird populations more resilient.”

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