Bird lovers and exhibitors watched on in horror as 38 prize budgies keeled over and died during a show.
Owners and organisers rushed about getting their cages outside into the fresh air as the budgerigars began plummeting inexplicably from their perches.
Fanciers feared a gas leak caused the tragedy.
But after investigations by plumbers, gas workers, fire authorities and environmental health officers, there was no explanation for what happened at the village hall event in Gwynedd, North Wales.
Retired pet shop owner Dave Cottrell, 55, who lost 10 birds at the show, said: "I was preparing the certificates as the birds were being put in order when a steward came over and said he had a dead one.
"Then another steward came over said he had two or three dead.
"And within seconds a third steward said he had more dead."
Mr Cottrell, who has 200 birds and been a fancier for 35 years, thinks a boiler or oven flue could have been temporarily blocked, forcing out fumes.
Robert Hughes, 34, - who organised the annual Gwynedd Budgerigar Society Open Show - said: "The birds were spread across the hall.
"Five minutes after the first one died eleven more had gone, one was mine.
"We made the decision to get everything out of the hall because we had 350 cages so we saved a lot of birds."
Small birds are especially vulnerable because of their tiny, sensitive respiratory systems.
Canaries were traditionally used by miners to test whether there were dangerous levels of carbon monoxide below ground.
Mr Hughes said: "Nobody in the community has seen nor heard anything like this before. The hall has been given the all clear.
"It's bizarre. It has been really difficult to get over it.
"It takes a year to organise the show and it's a lot of work - but that's nothing compared to how you feel for some of the people who lost birds."
A vet carried out checks on two casualties revealing they died from congestion and haemorrhaging of the lungs.