The speckled wood is one of a handful of species that appear to be benefitting from climate change, recently colonising East Anglia, the Midlands and much of northern England, increasing in abundance by 84% over the past 40 years. The southern population has expanded northwards at an average of four miles a year since the 1970s.

The butterfly has also ventured into the Irish Sea to reach the Isle of Man and spread across Scotland, where in the 1970s it was restricted to the mild west coast and the Moray firth. It may even be about to establish itself on the Outer Hebrides.

As part of this year’s Big Butterfly Count, Butterfly Conservation and the Tree Charter - a campaign to help protect the UK’s woodlands and wildlife - are asking the public to visit a local wood and record the speckled woods and other butterflies they see.

Results from the Big Butterfly Count, the world’s biggest annual insect count, will help scientists to understand why the speckled wood has thrived when three-quarters of Britain’s 59 native butterfly species are in decline.

“It’s a species that’s doing well but it’s also doing interesting things,” said Richard Fox of Butterfly Conservation. “Sightings in northern England, southern Scotland, Scottish islands and the far north are particularly important – you might discover a new area in which the speckled wood has never been found before. But we welcome sightings from everywhere so we can measure how well the speckled wood is doing nationally.”

The speckled wood, which flies in partially shaded woodland of dappled sunlight, appears to be a harbinger of a much milder and wetter climate.

According to lepidopterists, woods traditionally shelter speckled wood caterpillars from the harshest winter weather and the driest summer weather, when trees provide vital shade for the grasses on which the caterpillars feed.

When conditions are mild and damp enough, such as in its traditional stronghold of Devon, the speckled wood can move beyond woodlands and live in leafy lanes and gardens.

As climate change brings milder, damper conditions to eastern and northern Britain, the speckled wood has expanded.

Scientific studies of the butterfly have revealed that individuals living in newly colonised areas have developed larger thoraxes – containing their flight muscles – increasing their flight strength and making them more successful at dispersing into new territory. There’s an evolutionary tradeoff, however: these muscular migrants lay fewer eggs than the more sedentary speckled woods living at the core of its traditional range.

Last year, just 4% of Big Butterfly Counts took place in woodland, which are important for many butterflies, including threatened species such as the wood white, black hairstreak and pearl-bordered fritillary.

“Woodlands are vital for many butterfly and moth species as well as being beautiful places to explore and discover nature,” said Fox. “This summer let’s all try to do an extra ‘count’ in a local wood. A woodland count will help us understand how common butterflies are faring in our woods.”

Sir David Attenborough, the president of Butterfly Conservation, said: “The sight of a specked wood flitting through the dappled sunlight of woodland glade is a memorable high-summer spectacle.

“Sadly, our woodlands face a multitude of threats from habitat loss to climate change and various tree diseases. Spend 15 minutes counting butterflies in your local woodland and help us build a picture of how we can help protect woodland butterflies.”