Ravens and other members of the corvid family (crows, jays, and magpies) are known to be intelligent. They can remember individual human faces, expertly navigate human environments (like trash cans), and they even hold funerals for their dead.

Some ravens, it seems, even know the art of making a deal.

A new paper published in the journal Animal Behavior tested how well a raven could judge a deal that was "fair" or "unfair."

The study was conducted by researchers from the Lund University in Sweden, the University of Vienna in Austria, and others. Co-author Jorg Massen is a post-doc at the University of Vienna specializing in cognitive biology.

"This sort of cooperation is a part of [a corvid's] complex social life," said Massen. Understanding how they make choices, he explained, "gives us insights into the evolution of intelligence."

The ravens for the study were hand-raised by Massen and other researchers. The ravens were raised in captivity to make them less fearful of people and so they could be trained to trade tokens for rewards.

Using nine ravens, researchers taught the birds to trade a small crust of bread for a more tasty piece of cheese. In the first "fair" phase of the experiment, a trainer at one end of a cage gave a bird a piece of bread crust, which it carried in its mouth to the opposite end of the cage—where it was offered to a second researcher's outstretched hand in exchange for cheese.

In the second "unfair" phase of the experiment, the raven followed the same procedure, but the trainer then ate the morsel of cheese instead of exchanging with the raven.

After two days, the procedure was set up with the "fair" trainer, the "unfair" trainer, and a third neutral trainer. Of seven birds tested, six chose the fair trainer and one chose the neutral trainer. After a month, all nine were tested, during which seven chose the fair trainer, one chose the unfair trainer, and the last chose the neutral trainer.

Almost all of these training phases were conducted with a second raven in the cage to serve as an observer; however, birds that simply observed a different bird's interaction did not seem to be influenced in their own decision-making.

Though it wasn't tested, Massen believes the ravens would be capable of remembering fair and unfair trainers for as long as two years, the amount of time he says they're capable of remembering cage mates.

In the wild, where ravens don't rely upon a bread and cheese economy, the study offers insights into how corvids' complex social structure evolved.

"If one individual supports another, there's a correlation between support given and received on a long-term basis," said Massen. In other words, ravens build up social capital that is reciprocated over time. Favors in the form of preening or aid during a fight are selectively given to ravens in good standing with one another.

The extent to which ravens may differ based on individual personalities is a question the researchers hope to find an answer to next.