A butterfly species in England is expanding its range, thanks to climate change.

In the current issue of Science, researchers at the University of York report that the brown argus butterfly has spread its reach in England northward by about 50 miles over 20 years as a warmer climate allows its caterpillars to feed off wild geranium plants, which are widespread in the countryside.

“There was something unusual about the degree to which it was spreading its range,” said an author of the study, Jane K. Hill, a biologist at York. “It was turning up in places that were unexpected.”

The challenge was figuring out whether the butterflies were truly responding to climate change or shifting their range for some other reason. The researchers looked at data on butterfly locations and populations gathered by thousands of volunteers over four decades.