Scientists are on high alert for a new influx of bird flu as seabirds return to the UK coast in large numbers for the nesting season.
A second summer of mass mortality would have a devastating effect on the population of gannets and some other species.
And it would also raise the risk of the virus being transferred to mammals.
In an exclusive interview, Professor Ian Brown, the director of scientific services at the government’s Animal and Plant Health Agency (APHA), told Sky News that the source of the virus behind last year’s die-off was still unclear.
“It looks like the virus was introduced into seabird colonies by another population,” he said.
“So rather than those gannets going out to sea and being exposed to the virus, it’s more probable they were exposed to other birds that mingled in their colony.
“Gulls, which we know are very susceptible to this virus, can carry it and can shed it. So in theory they can introduce it into a colony.
“Once it gets into the colony, those birds are really tightly packed and the virus spreads very fast.”
The H5N1 virus caused thousands of deaths in seabirds last year.
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After spending the winter out at sea, gannets, guillemots, puffins and kittiwakes are beginning to congregate in large numbers on steep cliffs to raise their young.
“Hopefully those birds will come back and will have a healthy breeding season and they won’t be exposed to virus,” said Prof Brown.
“But knowing what happened last summer, we’ve got to be more watchful and look for early events that signal maybe the virus has come back.”
Volunteers and wardens for the Royal Society for the Protection of Birds (RSPB) are checking colonies for sick birds and scanning the sea surface for any that have died.
Half a million seabirds nest at Bempton Cliffs nature reserve, on the Yorkshire coast. So many gannets gather here that it is internationally important for the species.
But in the most crowded sections of the colony 80% of gannet chicks died last year.
Dave O’Hara, the RSPB’s senior site manager, said they almost certainly starved.
“We presume that’s because one or both of the adults had died,” he said.
“Things which are killing adult birds are of a particular concern because they only have one chick a year. So it could take a long time [for the population] to recover.
“This year, there’s a lot of worry about what’s coming.”
The H5N1 virus was first detected in 1996 and until two years ago was generally causing small outbreaks at certain times of year in domestic poultry, and migrating ducks and geese.
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But a new variant called 2.3.4.4b has swept the globe. It is now widespread in wild birds and no longer just causes a seasonal disease.
It has also begun crossing species to infect mammals, including foxes, seals and dolphins in the UK. All are most likely to have died as a result of scavenging birds with the virus.
There’s no evidence of mammal-to-mammal transmission in the UK, though it has happened at a mink farm in Spain and possibly in sea lions in South America.
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But the APHA is closely watching a mutation called E627K, which has been identified in eight out of 10 samples taken from UK mammals, and seems to increase the virulence and ability for airborne transmission.
“What we’re tracking are some changes in the virus that are rarely observed in bird populations,” said Prof Brown.
“They’re likely to signal that the virus is making some adjustment. But one adjustment alone will not be enough for that virus to then successfully transmit from one mammal to another.
“A numbers game is a factor. The more exposures you get, the more risk that these events might spark and trigger something that can then transmit, which is why we’re doing the surveillance.”
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There have been seven cases of the new strain in people working closely with birds, including one in the UK. All those in the US or Europe had mild or no symptoms.
The UK Health Protection Agency says the risk to the general public is low and there has been no human-to-human spread.
But it warns people should not touch dead or sick birds, and should wash their hands after feeding birds.
