1.Spedding and his colleagues think that the unique aerodynamic wake signatures of bat wings are caused by different mechanical operations in the upstroke of the wingbeat.
2.Hummingbirds however, turn their wrists during the upstroke, changing their wing's angle to flap in a figure 8 pattern that continually generates lift throughout their wingbeats.
3.Their wing joints are also stiffer than those of flying birds, which helps them produce thrust on both the downstroke - like most birds do - and on the upstroke of the wing.