The test is simple in outline. Food is hidden under one of two opaque cups. The experimenter shakes one cup: if it rattles, the reward is inside; if it stays silent, the reward must be under the other cup. Choosing correctly after hearing only the silent cup requires a step of negative inference rather than direct evidence.
Griffin, an African grey parrot at Harvard, performed above chance on exactly that condition in work published in 2019 by Irene Pepperberg and colleagues. Grey parrots had already been credited with vocal labelling, categorisation and some numerical competence through decades of work with the parrot Alex, who learned to use dozens of English labels for objects, colours and shapes before his death in 2007.
The usual misreading is that this shows parrots understand language. It does not. Labelling and exclusion are separate abilities, and much of the evidence rests on small numbers of intensively trained individuals, which is a real constraint on how far the findings generalise.