Spring has only just begun, according to the calendar, but some of the season's common events have been taking place weeks early, experts said.
After the UK's third-warmest winter in records going back to 1910, the Woodland Trust's Nature's Calendar has seen some of its earliest records this century for flowering bluebells and blackthorn.
Hawthorn trees have also been coming into leaf much earlier, according to the scheme in which members of the public record signs of the changing seasons.
This time last year, the trust had received 22 bluebell sightings across the UK, but more than 60 locations for blooms have been recorded this year, stretching from Dorchester to Doncaster.
There were 27 sightings of blackthorn flowering before the end of February last year, with 91 sightings up to the same point in the benchmark year of 2001, but this year there were 200 records.
This year, the Woodland Trust had received 244 sightings of hawthorn leafing by the end of February, compared with 40 sightings in 2015 and 35 sightings in the benchmark year.
But while plants and trees seem to be responding to the warmer conditions by getting going ahead of spring - which, according to the astronomical calendar, kicked off on March 20 this year - birds and insects do not seem to have done the same.
Birds such as blackbirds and blue tits are behaving much as normal, as day length is an important factor in when they breed, while sightings of insects including ladybirds and red-tailed bumblebees are also around average.
This could be because while December was the warmest on record, January and February were much closer to the average, and cold in some places, according to Kate Lewthwaite, the Woodland Trust citizen science manager.
She is not surprised by the sightings of plants and trees flowering and leafing early, she said, as it chimes with her own experience spotting the first flowering daffodil on Boxing Day, and the first snowdrop before Christmas.
She said: "Records which trace spring's arrival date back several centuries, but in the last 30 years we have seen a marked advance in the appearance of many spring signs. This year's particularly mild weather seems to have had a significant effect."
She added: "It's always that difference between weather and climate, you could say we've just had a mild winter. But increasingly we are having these mild winters.
"I think this is a warming trend we're seeing and we would expect species to adapt their behaviour accordingly."
The warmer conditions may suit some species but not others, and could lead to a mismatch between wildlife and their food sources, such as breeding birds and the insects they rely on.
The next signs of spring to look out for are flowering hawthorn and elder, while the peak bluebell season is still to come.
The Woodland Trust is looking for people to record bluebell sightings this year through its Big Bluebell Watch.