Not Before 2041: What Bird Flu Did to Britain's Gannet Colonies
Two of the largest Northern Gannet colonies on earth sit within sight of British coasts: Bass Rock, a plug of volcanic rock off Scotland's east coast, and Grassholm, a low island off Pembrokeshire. In 2022 highly pathogenic avian influenza reached both. New modelling says neither colony will be back to where it should have been until 2041 — and probably later.
What the numbers did
The study, led by the RSPB with Aarhus University, measured adult survival rather than counting corpses, which is why its results are hard to argue with. In the eleven years before the outbreak, average annual adult mortality was 6% at Bass Rock and 11% at Grassholm. During the outbreak year it rose to 33% and 47% — a fourfold increase in the death rate of adult birds.
Adults are the engine of a seabird colony. Gannets do not breed until they are several years old, they raise a single chick, and they live for decades; the loss of a breeding adult is not replaced next spring. By 2023, the year after the outbreak, the colony at Bass Rock was 26% smaller and Grassholm's was 38% smaller. Population modelling puts full recovery, relative to a no-outbreak scenario, no earlier than 2041 — nineteen years after the event.
Why this one is different
H5N1 was first identified in poultry in Asia in 1996, and the lineage that arrived in Europe in 2021 — clade 2.3.4.4b — behaved unlike previous versions: it did not fade away in summer, and it spread through wild populations across continents rather than appearing in isolated die-offs.
Gannets were among the worst hit. The 2022 outbreak was the first to be confirmed at Bass Rock on 4 June, and it went on to kill tens of thousands of birds. The same period produced similar numbers elsewhere: across north-western Europe, more than 20,500 adult Sandwich Terns were found dead within two months, over 17% of the entire regional breeding population, and almost every chick in the affected colonies died. In south-east Sweden, black-headed gull colonies lost 90–100% of their chicks; one colony was abandoned altogether and was still empty three summers later.
The Swedish work added something quieter and more disturbing: birds that survived the outbreak moved 27% more slowly each day and started migration earlier, and one colony's survivors still carried neutralising antibodies two years on. The virus does not simply remove birds from a colony; it rearranges what the survivors do.
The immunity question
There is a reason the story is not uniformly bleak. In the Sandwich tern colonies, 25.7% of adults tested after the outbreak peak carried immunity to H5, and in the Swedish gull colonies survivors still showed high antibody levels one and two years later. Some populations, in other words, are now partly protected — but immunity is not inherited as a guarantee, and it fades with the arrival of new virus variants. What that buys is time, not safety, and the modelling that puts gannet recovery at 2041 assumes no repeat on the 2022 scale.
What actually helped
One practical finding has changed how colonies are managed during outbreaks: removing carcasses reduced total mortality by roughly 15%, because it interrupts the cycle in which live birds peck at, copulate with or feed beside dead ones. It is unglamorous work and it works.
The wider seabird picture
The gannet's IUCN status is still Least Concern globally, and the study's authors say that should now be revisited — which is a measure of how unusual this outbreak was. Britain's seabird baseline was already poor: of the 26 seabird species currently breeding in the UK, ten are Red-listed in the 2024 Birds of Conservation Concern review, and 62% of UK breeding seabird species are in decline, rising to 70% in Scotland. Layered on top of that, a single virus season becomes a decade-long debt.
What to watch
Whether the counts at Bass Rock and Grassholm keep improving at the modelled rate, and whether H5N1 returns in a form that spreads as efficiently through dense colonies. The RSPB's recommendation is unspectacular but essential: keep monitoring the big colonies and the wider populations, because the only way to detect the next wave early is to have been watching before it arrives.
Listening to a gannet colony
Gannet colonies are loud in a way that has to be heard to be believed — thousands of birds giving a harsh, grating contact call, with the noise carrying kilometres out to sea. Here is the voice of the species at the centre of the story, followed by two gulls that share its waters and its outbreaks.