Some birds, i.e. swifts, are so specialised that they never deliberately land on the ground and apparently only come down at all in order to nest. It is reported that they eat, mate and sleep on the wing. They also migrate between Europe and Africa so it is highly likely that they do occasionally enter cloud whilst asleep (rather like some pilots I know)!
If they do come to earth they cannot take off again because their legs are so short and under-developed. They must land on a vertical surface such as a cliff face and launch themselves downwards to become airborne again. All their toes face forwards to allow them to grasp vertical surfaces, unlike other birds which have two forwards and two backwards or three forwards and one backwards.
I got to handle one of these fascinating birds for a few minutes many years ago, when I rescued it from our loft after it fell inside. It had been there for more than a day (my mother thought we had a rat up there and I was sent up through the trap-door to deal with it). I remember being surprised how big it was.
Initially I put it on the back lawn and at first I thought it was injured because it just scrabbled around. However, after I hand-launched it, it proved it was absolutely fine and it circled around my head for quite a while as if to acknowledge its thanks.
Yesterday I saw quite a few around and above cloud, in heavy rain at about three thousand feet.
ShyT