Thousands of people have signed a petition to save Britain's barn owls as 2013 was deemed the bird's worst year ever.
The number of barn owl nests varied between 45% and 95% lower than normal last year, the Barn Owl Trust said.
Changing climate and habitat loss is partly to blame, but they are also being killed by rat poisons used on farms across the country.
The latest research shows that 84% of Britain's barn owls feed on poisoned prey, and some die as a direct result, the Trust said.
Non-lethal doses of poison could also be stopping them from being able to breed and hunt properly.
The Government is reviewing how these poisons are used.
About 80,000 people have signed a petition - to the Health and Safety Executive - calling for "the introduction of stronger controls on the use of powerful rodent poisons and clear labelling on packaging, as recommended by the Barn Owl Trust".
Rodenticides can be used without any training and product labelling is inadequate and misleading, the Trust said.
David Ramsden, co-founder and senior conservation officer at the Barn Owl Trust, said they are "very, very concerned" that the low-level contamination, which he says nearly all of them now have, is "affecting their survival".
Mr Ramsden is optimistic about the petition but said establishing last resort use for the stronger poison could be difficult, as he said it will be a challenge to persuade test controllers, farmers and gamekeepers to think about using other control methods such as traps before using strong poisons.
He said there is a "good consensus" that the labelling on poisons needs to change, but he said it also needs to be made "honest".
"We're concerned that the labelling needs to be made honest. A lot of the farmers we talk to have no idea that so many barn owls and kestrels and other birds are poisoned.
"And if the products' label actually said how many were poisoned a lot of them wouldn't buy it," he said.
Mr Ramsden said barn owls are not only beautiful but an "indicator of the health of the environment".
He said: "They're incredibly beautiful birds. They used to be very common. They're an iconic countryside species. People love to see them, and they're an indicator of the health of the environment.
"If something's going wrong, if the habitats aren't right or the climatic conditions aren't right, it shows up most of all in the predators, the ones at the top of of the food chain.
"So they're an indicator species."