There's also the common swift, who spends almost all its time in the air and absolutely never lands on ground, due to being unable to take-off -- its legs are t

There's also the common swift, who spends almost all its time in the air and absolutely never lands on ground, due to being unable to take-off -- its legs are too short and weak to do that. They only land on rock faces and take off by letting go. That's where they build their nests too. You can see them in a lot of cities actually -- high buildings are pretty much perfect for them to nest on.