Skeleton birds appear to be the skeletons of dead birds. They have two empty eye sockets in its skull with a big beak. The bird's neck is slightly bent upwards at the area where the skull is connected. The bones of the bird's wings are bent and exposed, connected to the bird's ribcage. The birds have short legs.
Touching a skeleton bird won't kill the player, but the eggs they lay will. This is probably because they are only skeletons, and only living enemies will hurt the player in this game.
The skeleton birds act more in the game as egg spawners, because touching them won't hurt the player, but what they lay will. The eggs they drop will shatter upon meeting the ground. The skeleton birds usually cannot be reached at all, as they are high in the sky, where the player cannot reach them. However, on the level A Width Of A Hair, it is possible to send the cactus men into the air and go through one bird.
Since the skeleton birds lay eggs, they are all female.
In some levels where skeletons birds are up ahead of the cactus men, if direction is changed the skeleton birds will disappear.