Insightful string-pulling in Grey parrots (Psittacus erithacus) is affected by vocal competence

Four Grey parrots (Psittacus erithacus) were tested on their ability to obtain an item suspended from a string such that mutiple, repeated, coordinated beak-foot actions were required for success (e.g., Heinrich 1995). Those birds with little training in referential English requests (e.g. I want X) succeeded, whereas birds who could request the suspended item failed to obtain the object but engaged in repeated requesting.