Crows hold a somewhat eerie status in our folklore. Perhaps inspired by their black plumage and coarse caws, stories and legends depict these birds as ominous creatures, messengers between the realms of the living and the dead, harbingers of death and misery. Crossing paths with a crow can be an unsettling experience, not least because it feels as though these highly attentive birds are scrutinizing us with their deep, penetrating gaze.

There’s no way to figure out what they actually think of us, if they think about us at all—but it’s clear that when it comes to intelligence, carrion crows (Corvus corone) sit at the head of the avian class. Birds in general, were once thought to be devoid of complex reasoning skills. Their brains lack a neocortex, the six-layered brain region that evolved in mammals and is implicated in higher-order cognition. What they do possess, however, is a group of nuclei, collectively known as the pallium, that have been proven to carry out neocortical-like higher cognitive functions. Crows in particular have continuously made behavioral scientists marvel at their learning ability and problem-solving skills—including a surprising ability to discriminate  quantities.

What exactly are the neural mechanisms behind this bird’s remarkable grasp of numerical quantities? This question inspired a team of researchers led by Andreas Nieder, at the University of Tübingen, Germany, to peer into the minds of crows. In a first study, published in Proceedings of the National Academy of Sciences in 2015, the scientists trained crows to perform a task that required the ability to correctly discriminate the number of elements in a picture. Each trial consisted of the sequential presentation of a sample and a test picture, both containing a random number of black circles, from one to five.

The crows successfully completed the task if they could identify the correct match between sample and test picture by pecking the touch screen, or by withholding that response when there was no match. The birds successfully identified matched and unmatched pairs of pictures in around 75 percent of the trials, and their accuracy improved as the numerical difference between the sample and test number of elements increased. In other words, they would be less likely to err if comparing two circles to five than if comparing two to three.

To explore the neural basis of this ability, the researchers used small electrical wires, surgically implanted in a region of the bird’s pallium, called the nidopallium caudolaterale (NCL), and recorded the activity of neurons while they performed the task—specifically during the presentation of the sample picture. They found that the activity of subpopulations of neurons in the NCL increased with the presentation of only one specific number of circles.

For instance, some neurons showed increased activity when the crows were presented two circles, and others after presentation of five circles. Curiously, the activity of these neurons changed as a function of the numerical difference between their selected quantity and an alternative one; that is, a neuron selective for one element showed the highest activity when a picture with one circle was presented, less activity when three circles were presented, and the least activity when five circles were presented.

The researchers also observed that when the birds made an error, the response of the neurons to their preferred quantity was not only significantly lower than when the bird was correct, but also more similar to their activity for a non-preferred quantity. This suggested that on error trials, an incorrect encoding by the neurons with a preference for the quantity being displayed had led the crow to make a judgement error.

These first results showed that crows can perform numerical judgements, much like primates and humans. However, in the world of birds, such an ability is not unique.  New Zealand robins, for instance, have been shown to preferentially collect food from caches containing greater amounts of mealworm, when simultaneously presented with alternatives of different amounts. What was more surprising was the fact that crows have quantity-selective neurons in their brains, whose activity seems to be linked to their ability to correctly perform numerical assessments—something that previously had only been shown in primates. 

The team wanted to further understand this faculty and the presence of quantity-selective neurons, and determine if these are inherent, or somehow require learning. In a follow-up study, published this year in Current Biology, the team trained a new set of crows in a color discrimination task. The crows had to correctly discriminate matched and unmatched pairs of pictures based on the color of the circles rather than their quantity. The crows mastered this task, and even though the task had nothing to do with assessing quantities, recordings of neuronal activity in the NCL revealed that quantity-selective neurons there were keeping track of the number of elements displayed in the pictures. Since the crows had not been previously trained on a numerical task, the finding provides strong evidence that the ability to discriminate quantities is inborn rather than learned.   

Together, these results show, for the first time, that a species of bird has inherent numerical abilities that, to some extent, put it on par with primates and humans. It is extraordinary that even though birds and mammals occupy two branches in the tree of life that are separated by 300 million years, and have evolved very different cognitive structures, their brains nonetheless share similar solutions for encoding quantities and possibly performing complex numerical reasonings. This link between the activity of specific neurons and behavior is one of the most fascinating aspects of modern neuroscience, and one that inspired me to study the neuronal mechanisms behind our memories and our ability to mentally represent space and use this for orientation.

Whether it’s crows assessing quantities or rodents using a cognitive map of their surroundings to navigate their environment, these experiments reveal important evidence that increases our understanding of how the coordinated activity of populations of neurons leads to the emergence of a wide variety of complex cognitive behaviors. In the end, crows might not be able to solve advanced mathematical equations. Nonetheless, discoveries such as these inspire awe and wonder. Learning how these birds, with their relatively simple brains, display such sophisticated reasoning skills, may in fact hold relevant clues about our own cognitive prowess.