Christian Schloegl and his team at the University of Vienna, let six parrots choose between two containers, one containing a nut. Both containers were shaken, one eliciting a rattling sound and the other nothing. The parrots preferred the container that rattled, even if only the empty container was shaken.
When the parrots were played a recorded noise, but the container was not shaken, they did not choose it. This suggests that grey parrots understand the causal link between nut, shaking and noise, which is otherwise only mastered by great apes, but not by monkeys or dogs.