(CNN) Only a few mammals -- humans and a few other primates -- are thought to spontaneously help one another.

Now, however, scientists say African grey parrots voluntarily help each other to obtain a reward of nuts. They say it's the first time such behavior has been seen outside of mammals.

"We found that African grey parrots voluntarily and spontaneously help familiar parrots to achieve a goal, without obvious immediate benefit to themselves," said study co-author Désirée Brucks, a researcher at the Max Planck Institute for Ornithology in Germany.

The researchers studied two types of parrot -- the African grey and the Blue-headed macaw. Both species were eager to trade a metal token with a researcher in exchange for a nut treat. However, only the African grey parrots were willing to transfer a token to a neighboring parrot, allowing the fellow bird to earn a nut reward.

This image shows an overview of the test setup: the bird on the left can exchange tokens, while the exchange hole on the right is covered.

"Remarkably, African grey parrots were intrinsically motivated to help others, even if the other individual was not their friend. So they behaved very 'prosocially,'" said study co-author Auguste von Bayern, also a researcher at the Max Planck Institute.

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