Twenty species, included chickens, were tested for the virus

The virus is transmitted by mosquitoes and the researchers have warned that the risk of the virus spreading to humans is increasing with the impact of climate change.

It can cause fatal inflammation of the brain in humans. There have been no cases of the virus in the UK but it killed more than 270 people in the US last year.

Scientists at the Centre for Ecology and Hydrology in Oxford tested birds mainly in Cambridgeshire, but also in Dorset and South Wales.

They found evidence of the virus in more than half the birds tested - an "unexpectedly high" proportion, BBC science correspondent Christine McGourty said.

It was found in more than 20 species in all, including crows, magpies, swallows, chickens, turkeys and ducks.

Migration

While the birds were healthy and showed no symptoms, scientists did detect antibodies to the virus.

This indicated the birds had come into contact with the virus and that their natural defences had successfully fought it off.

It is thought the virus is being brought into the country by migrating birds.

The researchers said there was no immediate threat to humans - but added the warning about climate change increasing the risk.

The research, published on Saturday, comes after increased surveillance measures for the virus were introduced in the UK.

The chief medical officer for England, Sir Liam Donaldson, announced the measures in early July, saying the risk to human health was low, but doctors and health officials had been urged to be on the look-out for symptoms.