Gannets’ blue eyes turn black after an infection with bird flu
The blue eyes of some seabirds appear to turn black after they have had a bird flu infection.
The colour change, seen in northern gannets (Morus bassanus), may give scientists a new way to track the impact of the virus outbreak.
Bird flu has circulated seasonally among wild and farmed birds for decades, but, since October 2021, a highly pathogenic strain of the virus has swept through wild and farmed bird populations with unusual virulence.
Seabirds in Europe and the UK have been particularly hard hit, with thousands killed in the past year by the H5N1 virus, including threatened gannets, puffins and great skuas.
The adult survival rate for the 150,000-strong gannet population on Bass Rock, an island off the east coast of Scotland, for example, was 42 per cent below average between 2021 and 2022.
Without performing invasive tests, scientists have struggled to tell whether seabirds have suffered infections and survived or so far escaped contact with the virus.
Gathering this information is crucial for better understanding how the virus is affecting wild bird populations, including assessing the survival rate and whether these birds are developing any immunity to the disease.
Gannets with black or mottled black irises, rather than the standard pale blue colour, have been spotted for the first time in several colonies known to have been affected by bird flu, including in the UK, France, Germany and Canada.
Jude Lane from the RSPB, a UK conservation charity, and her colleagues took samples from 18 apparently healthy gannets with normal and black irises living on Bass Rock. Eight of the birds tested positive for bird flu antibodies and, of those, seven had black irises.
The incidence of this trait could be a useful non-invasive diagnostic tool for conservationists tracking the impact of bird flu, says Lane. “To be able to look at how many birds are dying, but also how many birds are surviving, will allow us to add these details into population models to predict what populations of seabirds might look like in the future,” she says.
It is unclear why the irises turn black, but Lane and her colleagues are looking into this. She also plans to study whether the change is permanent, how long the virus antibodies persist in gannets and whether the birds suffer any adverse long-term effects from infection, such as fertility or vision problems.
It will also be crucial to understand whether the same changes to eye colour occur in other bird species, she adds, although the feature may be harder to spot in those with naturally darker eyes.