Apple seeds and the pits of cherries, peaches, plums nectarines and apricots contain a cyanide compound. They are dangerous to humans as well, although being much larger than our birds, we would have to eat much more than they do to become ill or even die. Just as with nut shells, birds can break off pieces of pits with their sharp beaks and it takes a lot less to make them sick. These pieces can also cause blockages in the digestive system if swallowed. Either cut the fruit away from the pit or keep on eye out and remove it when the bird finishes the fruit.