Black eyes indicate bird flu survival in Northern Gannets

A new study has discovered evidence that Northern Gannets can recover from Highly Pathogenic Avian Influenza (HPAI), with black irises being an indicator of a previous infection. 

Scientists from multiple organisations investigated outbreak timings at colonies across their North Atlantic range. At the largest colony, Bass Rock off the Lothian coast, a detailed study was conducted to estimate the impact of the virus on colony size, breeding success, adult survival and whether gannets were potentially able to recover from an infection. 

Black irises – instead of the usual pale blue – were first seen in Northern Gannets breeding on the Bass Rock in June 2022, with colour varying from completely black to mottled. The team took blood samples from 18 apparently healthy adult Gannets with both normal and black irises, which were tested for bird flu antibodies by APHA to determine whether the birds had been previously infected. Eight tested positive, of which seven had black irises.

Dark-eyed Northern Gannets have been shown to have recovered from avian influenza (Alex Penn).

Dr Jude Lane, RSPB Conservation Scientist and lead author of the study, explained: "This has been a fascinating development and the discovery may 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. Ophthalmology exams will also be needed to determine what is causing the black colouration."

HPAI has negatively impacted wild and domestic bird populations globally for decades. However, the current H5N1 strain has seen shifts in both the timings of outbreaks and species affected - including seabirds. Northern Gannets appeared especially severely impacted, but there was limited understanding of how their populations were affected.

High numbers of dead gannets were seen in Iceland during April 2022 followed by outbreaks in many Scottish colonies, Canada, Germany and Norway. By the end of June, outbreaks had occurred in five Canadian colonies and in the Channel Islands. Outbreaks in 12 UK 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) and sampling data was available for 58% of these, all with dead birds testing positive. 

To better understand the impacts of HPAI, the team further investigated Bass Rock, which is home to more than 150,000 Northern Gannets at its peak. The team calculated that adult survival between 2021 and 2022 was 42% lower than the preceding 10-year average. The full extent of how many birds died during that period won't be confirmed until all the birds return for the 2023 breeding season. 

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. 

Susan Davies, CEO of the Scottish Seabird Centre, said: "Like many Northern Gannet colonies across the North Atlantic the Bass Rock was severely impacted in 2022 by highly pathogenic avian influenza. Due to the long running research effort on the Bass Rock, it was possible to gain important insights into the changes taking place in the colony with a strong link emerging between virus infection and the changing iris colour in these striking seabirds and the high level of nest failure within the study area."

Seabirds are among the most threatened group of birds. Some 24 of 25 UK-breeding species are Red- or Amber-Listed on the UK list of Birds of Conservation Concern. The 2022 bird flu outbreak has provided another significant stressor to those already faced by our rapidly declining seabird populations. Quantifying and perhaps even mitigating its impact is therefore crucial if we hope to restore our seabird populations.