The early arrival of spring in the UK this year has left climate watchers "enormously concerned", a conservation charity has said.
The Woodland Trust, which in association with the BBC runs the annual Springwatch monitor, says that two key indicators of the approach of summer have already occurred in Britain.
Observers have noted hawthorn flowering already this year and swifts returning much earlier than usual, leading scientists to express concern about an increasingly rapid change in Britain's climate
Norman Starks, operations director for the Woodland Trust, said the recent variations to the usual British seasons were much more dramatic than changes seen in the past.
"We're enormously concerned," he told the Today programme.
"I think the thing that concerns us is the pace of change. Climate change has been happening gradually over a long period of time, but over the last ten years the pace of change has been really very quick and I think the environment is struggling to adapt to that."
Woodland Trust and the BBC are asking people to record when they first spot six key species in their areas in order to track the changing of the seasons.
Similar surveys in the past have led to more than 300,000 sightings and the conclusions are revealed on a TV series later in the year.
Hawthorn flowering and the arrival of swifts usually occur in May but both have been spotted a month early this year.
"Along with thousands of volunteers we've been measuring when swifts come to the country and the average is May 10th, but here we are on the April 18th and they're already here," Mr Starks added.
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