Probably no harder than the flying squirrels and flying foxes. Check back with them in 10 million years and I bet they will have evolved into a whole new species of flying mammal.
Kind of like how the bat evolved into a flying mammal.
Been watching birds in the yard lately. Some of them are not graceful at all. Most plan their moves but some just jump. Saw a gray jay fall on his beak when he started to take off from the porch and stopped short at the edge. The small birds don't fly so much as jump and grab. They'll end up hanging from a branch upside down very often. They probably learned to fly by climbing trees and jumping, which the squirrels do now whether they have airfoil or not.