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Crows can make decisions according to the likelihood of getting a reward 鈥� a cognitive feat known to mathematicians as statistical inference, and rarely found outside of primates.

鈥溾�楤ird brain鈥� is often used as an insult, but that鈥檚 not really the case,鈥� says at the University of Tubingen in Germany.

Her team trained two carrion crows (Corvus corone) to peck at nine, different-coloured symbols to receive a reward: a small food pellet or worms. Over hundreds of trials, they were taught that each symbol was associated with a different probability of receiving a reward, ranging from 10 to 100 per cent per peck. The crows were given the chance to choose between two options: for example, the green circle with a 90 per cent chance of receiving a reward versus the blue square with a 70 per cent chance. There were no wrong answers, just less optimal ones.

Both crows pecked on the shapes with the highest probability of yielding them a reward more than seven times out of 10. In a second part of the experiment, shapes with lower probabilities were shown more often. While the crows could have pecked them more times and still gotten the same amount of reward, they still chose the shapes with the higher probability of getting them a treat.

鈥淭hey knew that even though it was shown less often it was more valuable,鈥� says Johnston.

This suggests the crows have the ability to use limited information about the probability of something happening and apply it in a new situation to maximise the possibility of getting a reward, she says.

When tested a month later, the crows made decisions to peck the shapes with higher or lower probabilities at the same rate. 鈥淭hat鈥檚 pretty incredible,鈥� says Johnston.

More research could explore how crows fare when it comes to keeping in mind various bits of probabilistic information at the same time out in the real world 鈥� say, if they have a 60 per cent chance of finding food by foraging in one site, compared to a 30 per cent chance in another site where their chances are also dependent on the rain.

These findings add to 鈥渁 growing body of fascinating evidence on remarkable cognitive capacities鈥� in corvids, says at the University of G枚ttingen, who was not involved in the research.

Studies have suggested crows can grasp the concept of something most human children don鈥檛 learn before around age 3.

Rakoczy says that, prior to this, such complex numerical abilities were primarily known in primates. This not only highlights how brains can be very physiologically different and still carry out similarly complex reasoning, but also how the latter has cropped up independently in completely different branches of the evolutionary tree, he says.