Grey squirrels could have taken over from native red squirrels in the UK due to their problem-solving powers, researchers have said.

A study tested wild squirrels with two tasks, one easy and one more difficult, to get hazelnuts.

Both species were equally successful at the easy task - opening a transparent lid - but more of the grey squirrels cracked the difficult one, a more complex task of pushing and pulling levers.

Researchers from the universities of Exeter and Edinburgh said this "superior behavioural flexibility may have facilitated their invasion success".

Grey squirrels, which arrived from North America in the 19th century, now outnumber red squirrels by more than 15 to one.

Dr Pizza Ka Yee Chow, from the University of Exeter's Centre for Research in Animal Behaviour, said: "Many factors have been considered to explain why grey squirrels are more successful when they move into areas where red squirrels live.