African grey parrots are no bird brains - they can not only learn to talk, but they have outperformed human two-year-olds in a test of intelligent reasoning.

No other animals apart from great apes match the birds' ability to understand noise-related causal connections, say scientists.

Human children only do as well as the parrots from about the age of three.

Researchers tested six African greys housed in a parrot rescue centre in Vienna, Austria.

During a series of experiments, the birds were asked to choose between two closed boxes, one of which held a piece of walnut and rattled when shaken. The other, empty container, could be shaken without making a noise.

The parrots showed they knew how to detect hidden food rattling in a shaken box.

ALSO SEE: Crows Are As Clever As Young Children, Says University Of Cambridge Researcher

But much more impressively they also worked out - almost instantly - that if a box was shaken and made no noise, the food must be in the other container.

Choices were made by a parrot walking over to a box and turning it over with its beak.

In similar tests, most animals - and even small children - get confused about the way shaking and noise relate to the presence or absence of a hidden reward.

The scientists, led by Dr Christian Schloegl, from the University of Vienna, wrote in the journal Proceedings of the Royal Society B: Biological Sciences: "We found compelling evidence for the ability of African grey parrots to use noise created during the shaking of containers to detect hidden food.

"Even from the very first trial, our subjects could also use the absence of noise in a shaken container to infer that food must be in the other, non-shaken container."

"Such behaviour has so far been shown only in the great apes, but not in any other non-human animal."

They added: "Human children solve this task from an age of three to four years, and the birds' success rate was comparable to those of the three-year-olds.

"The parrots' near-perfect first-trial performances as well as the results of our control experiments suggest that an understanding of the causal properties underlying the task is the most likely explanation for the birds' performance."

The parrots could not be tricked by playing them recorded sounds of a box rattling, said the researchers. If a box was not shaken at the same time, it was ignored.

It was "remarkable" that the African grey parrots, which were not used to taking part in studies, were able to out-perform highly experienced monkeys, the scientists wrote.

An odd result from the experiment was that the parrots appeared sensitive to side-to-side but not up-and-down shaking. The researchers speculated that vertical shaking may have distracted the birds by reminding them of head-bobbing, a common parrot behaviour.