Crows have a reasoning ability rivalling that of a human seven-year-old. They can recognise faces, use tools – even drop nuts on a road to crack, then pick them up with the safety of pedestrian lights.
Birds in the family of Corvidae – mainly crows and raven– have long been known for their intelligence. Even ancient Greek storyteller Aesop wrote a fable in which a thirsty crow drops stones to raise the level of water in a pitcher. To put this story to the test, scientists subjected six wild New Caledonian crows (Corvus moneduloides) to a battery of tests designed to challenge their understanding of cause and effect.
In the 'water displacement task', the crows worked out how to catch floating food rewards by dropping heavy objects into water-filled tubes. They demonstrated an ability to drop sinking rather than floating objects, solid rather than hollow objects, to choose a high water level tube over one with low water level, and a water-filled tube over one filled with sand. The crows failed on two more difficult tasks, however. One test required understanding of the width of the tube and the other involved displacing water in a U-shaped tube.
The New Caledonian crow is, so far, the most intelligent of its crow brethren. Proficient toolmakers, New Caledonian crows use sticks and serrated palm fronds to dig out insects from inside trees. But other members of the corvid family are also pretty brainy. And corvids don't even need instructions to get a job done. A 2014 Current Biology study showed they can work problems out spontaneously. So why are they so smart?
Crows tend to have big brains compared to their body size. In mammals, the bigger-brained are humans and dolphins; in birds, it's parrots and crows. And getting into the neuroscientific nitty gritty of it all, the part of the brain responsible for crow smarts is the "nidopallium caudolaterale". Two studies published in the Proceedings of the National Academy of Sciences last year showed just what the nidopallium caudolaterale can do.
Andreas Nieder's laboratory from Germany's University of Tübingen recorded signals from individual neurons in crow brains. One of their studies found neurons in the nidopallium caudolaterale change during a learning task, while in another crows were presented with varying numbers of objects, such as four dots. Neurons in their nidopallium caudolaterale seemed to be "tuned" to different numbers.
In mammals, the equivalent brain area is the prefrontal cortex, which is responsible for executive functions such as social behaviour and decision making. (The nidopallium and prefrontal cortex are examples of convergent evolution: some 300 million years ago, birds and mammals split, long before any sophisticated brain structures such as the nidopallium existed. And over time, they evolved independently.)
So why this incredible bird brainpower? Surely, in the wild, a crow doesn't need to know how to count. And the brain's a fuel-guzzler, so any increase in size needs to be worth the extra energy.
More information: Theory of a monkey mind
While it might have something to do with the fact that they are omnivorous, which means they feed on a variety of fruits, vegetables and meat, the leading theory is what's called the social intelligence hypothesis. Chimpanzees, for instance, have their own brand of politics, where not only do they have to keep tabs on their own relationships, but also those between others. That "social intelligence" needs a lot of brain power.
Many species of crow pair off as adults and become sedentary and territorial mates for life. But during younger years, they often live in larger flocks dubbed juvenile gangs, basically a bunch of teenagers trying to compete with each others. And it's these relationships they have to keep tabs on. A side effect of this is they can also keep tracks of their relationships with people. So the next time you shoo away a crow, remember: it's making a mental note of the experience and may one day get its revenge.
Jackdaws (Corvus monedula) are closely related to crows but their eyes are almost white, in sharp contrast with their dark feathers. These birds have been shown to use them as a warning signal to successfully deter competitors from coming near the cavities in trees that they use to nest in. The whiteness of human eyes is almost unique among primates and is considered to be important for our social world. A theory, called the cooperative eye hypothesis, proposes that the advantage of having white sclera helps us to follow the gaze of others more easily.