A type of hazard exclusive to the Library floors. They resemble pterodactyl skeletons which will be lying on the floor. Shortly after Cordy enters the room, they will rise and start chasing her in a manner similar to ferals, including dealing contact damage. Dealing any damage to them will force them to collapse again for short time. While collapsed, they won't do any damage. They will continue attacking even after all of the enemies in the room are killed or even if there weren't any there to begin with. Only prop-breaking attacks can dispose of them permanently.
Because the Trapstem Roses's hitbox is slightly larger than Cordy herself, it basically offers immunity from bonebirds. With it equipped, bonebirds can damage Cordy only if she is standing on it when it starts rising.