It is an unexpected influx of migrants that has the natural world a-flutter. Half a mile up, and moving at the speed of a small car, a cloud of perhaps 10 million is on its way from Africa, via the Arctic Circle.
The once-in-a-decade invasion of the painted lady is a phenomenon beloved by naturalists but little known outside the arcane world of lepidopterology.
However, with emerging signs that this will be a summer for the ages, a campaign is being launched to make social butterflies of us all.
The wildlife TV presenter Chris Packham is behind what is billed as the world’s largest insect citizen science survey, to gauge whether painted ladies are indeed arriving in their millions on the UK’s shores this year.
The insect is a common annual immigrant from the Continent to Britain, where its caterpillars feed on thistles. But around once every 10 years there is a painted lady “summer” when clouds arrive en masse. The last was in 2009 when around 11m painted ladies descended on the country.
The wildlife charity, Butterfly Conservation, which has for the last decade run an annual Big Butterfly Count, said unusually high numbers had been reported across Europe during the spring and early summer, with large numbers now spotted crossing to the UK.
More than 400 sightings have already been reported across Yorkshire, including at least 100 from sites within the North York Moors and more than 50 in the Yorkshire Dales.
Another 60 records have been sent in from areas close to Harrogate, and there have been numerous painted lady sightings near Leeds, York and Middlesbrough.
Mr Packham, who is vice president of Butterfly Conservation, said the painted lady migration was “one of the wonders of the natural world”.
He said: “Travelling up to 1km in the sky and at speeds of up to 30 miles per hour these seemingly fragile creatures migrate hundreds of miles to reach our shores each year.
“This butterfly undertakes an extraordinary 7,500-mile round trip from tropical Africa to the Arctic Circle every year – almost double the length of the famous migrations of the monarch butterfly in North America.
“Signs across Europe are looking very promising, meaning that 2019 could be a very good year for the painted lady with high numbers already being recorded across parts of the UK.”
The painted lady, whose scientific name is vanessa cardui but is often referred to as the thistle butterfly, is distinguished by its deep orange, black-spotted wings. It can turn up anywhere, and city and town spotters are being encouraged to take part in the survey as well as those in the countryside – either in back gardens, local parks or while out walking the dog.
They are asked to spot and record 17 species of common butterfly, including painted ladies and two day-flying moths, during three weeks of high summer, to help scientists see how the species are faring.
Experts also point to the beneficial effects for those doing the counting, with research showing that watching wildlife can improve mental health and well-being.
Mr Packham said: “The mental health benefits of spending time outdoors watching nature have been blindingly obvious to me for as long as I can remember.
“Immersing yourself in nature, even if it’s just for a few short minutes, changes your perspective. It helps you slow down and notice what’s going on around you and it opens a door to the overlooked beauty and drama of our natural world.”
The Count runs from today until August 11, with a website on which to record findings, and a phone app available to help keep count.