غداف شائع

Recent studies suggest that under experimental conditions, rooks will drop stones in pitchers of water much as they do in Aesop's fable. Researchers placed a worm in a narrow tube, out of reach of the bird. Four different individuals solved the problem independently, and three learned quickly to use larger rather than smaller stones.

http://www.cell.com/current-biology/abstract/S0960-9822(09)01455-9