In an experiment, two ravens had to simultaneously pull the two ends of one rope to slide a platform with two pieces of cheese into reach. If only one of them pulled, the rope would slip through the loops, leaving them with no cheese. Without any training they solved the task and cooperated successfully.
However, when one of the two birds cheated and stole the reward of its companion, the victims of such cheats immediately noticed and started defecting in further trials with the same individual.
“Such a sophisticated way of keeping your partner in check has previously only been shown in humans and chimpanzees, and is a complete novelty among birds.”