The vivid blue irises of northern gannets turn black if they survive avian flu, according to a study which provides evidence that some wild birds are shaking off the deadly virus.
Avian flu has killed wild and domestic birds for decades but the current strain (H5N1) severely affected seabird populations across the North Atlantic last year, with particularly high death rates among gannets.
Scientists from several organisations investigated the timings of avian flu outbreaks at colonies across their North Atlantic range, with a detailed study of the impact of the virus on the Bass Rock, Scotland, home of the largest gannet colony in the world.
Black irises – instead of the usual pale blue – were first seen in gannets breeding on the Bass in June last year, with colour varying from completely black to mottled.
Researchers took blood samples from 18 apparently healthy adult gannets with both normal and black irises. The samples were tested for bird flu antibodies to determine whether the birds had been infected previously. Eight tested positive, of which seven had black irises.
Dr Jude Lane, a conservation scientist for the RSPB and lead author of the study, said the “fascinating” discovery that gannets that had survived avian flu infection no longer had the usual iris coloration could prove a useful non-invasive diagnostic tool.
“The next steps are to understand its efficacy, if it applies to any other species and whether there are any detrimental impacts to the birds’ vision,” she said. “Ophthalmology exams will also be needed to determine what is causing the black coloration.”
High numbers of dead gannets were first spotted in Iceland in April last year. By the end of June, bird flu outbreaks had occurred in five Canadian colonies and in the Channel Islands. Outbreaks in 12 British and Irish colonies followed in a clockwise pattern with the last infected colonies recorded in September. Unusually high mortality was recorded at all but one of the 41 monitored colonies (75% of the 53 North Atlantic colonies of gannets).
On the Bass, the researchers calculated that survival rates of adult birds between 2021 and 2022 was 42% lower than the preceding 10-year average. The full extent of how many birds died during that period will not be confirmed until the birds return this breeding season.
The Farne Islands, a popular archipelago of seabird breeding islands off the Northumberland coast, are closed to visitors because of new outbreaks of avian flu this spring.
The study was a collaboration between the RSPB, the University of Glasgow, the University of Edinburgh, Heriot-Watt University and the Animal Plant Health Agency, in partnership with the Scottish Seabird Centre.
• The headline of this article was amended on 4 May 2023. The surviving gannets’ irises turn from blue to black, not black to blue as stated in an earlier version.
