Credit: S. DALTON/NATUREPL.COM

Humans, chimpanzees and other great apes can infer the presence or absence of hidden objects using even indirect evidence. Christian Schloegl and his colleagues at the University of Vienna tasked six African grey parrots (Psittacus erithacus; pictured) with determining which of two boxes obscured an object after they had witnessed one of them being rattled. Without the need for training, the parrots picked the correct container at rates above chance, even when the empty container was shaken and the birds had to use the absence of a sound to guide their decisions.