"The blossoming of hawthorn and return of migrating swifts - usually in May - have prompted the Woodland Trust to declare 'spring is the new summer'.
"The trust has been analysing reports from members of the public taking part in the BBC Springwatch survey."
It said it showed how climate change was affecting wildlife and seasons.
The return of the swift from warmer countries, usually expected in mid May, could mean summer has come early, said environment correspondent Sarah Mukherjee.
Sightings of swifts have been reported across south-east England and Gloucestershire over the last few days.
Swifts usually return from migration around 10 May when temperatures are slightly milder.
The flowering of hawthorn, normally around the middle of next month, is considered to signify the start of summer.
This year, the earliest flowering hawthorn was recorded on 9 April.
Dr Kate Lewthwaite of the Woodland Trust said: "Thanks to the exceptionally mild start to the year we are experiencing, this summer signal is arriving three weeks earlier than the 11 May average."
Dr Lewthwaite added the timing of the seasons was changing, with the mild winter and warm spring contributing to warmer than average temperatures.
"It is an example of how climate change is affecting our wildlife and seasons."
Springwatch - a survey into the arrival of spring - asks members of the public to record the first sightings of the year of six species - frog spawn, seven-spot ladybird, red-tailed bumblebee, peacock butterfly, flowering hawthorn and swifts.
Experts at the Woodland Trust said the sightings of hawthorn flowering were accurate and had been verified.