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Crows can recognise themselves in mirrors, use tools and plan for the future, all cognitive abilities more similar to those seen in non-human primates than to those of most other birds. This intelligence may be related to them having an unusually high number of brain cells involved in processing information.

at the Ruhr University Bochum in Germany and his colleagues analysed the brains of common ostriches (Struthio camelus), brown warren chickens (Gallus gallus domesticus), racing homer pigeons (Columba livia domestica) and three members of the corvid family: carrion crows (Corvus corone), hooded crows (Corvus cornix) and rooks (Corvus frugilegus). The animals had all been killed for food or pest control.

The researchers were able to analyse the nuclei of the all the birds鈥� brain cells using a method called isotropic fractionation. This allowed them to categorise the types of cells present in each brain and estimate how many there were of each.

The team found that corvids had the highest number of interneurons, small cells that pass on local signals and are involved in cognitive processing. These cells process information received from sensory neurons and send inputs to motor neurons. They are involved in tasks such as decision making, future planning and risk assessment.

鈥淢any studies have shown that different subsets of interneurons are extremely important in behaviours that are termed 鈥榠ntelligent鈥� in both mammals and birds,鈥� says Str枚ckens.

The corvids each had about 290 million interneurons, compared with 124 million in ostriches and around 40 million in the pigeons and chickens. Humans have about 1.3 billion interneurons. The difference was especially stark between ostriches and corvids because the brain of an ostrich weighs nearly double a crow鈥檚 brain.

鈥淚f we think about the neuron as the main processing unit of the brain, we can assume that a higher number of neurons equals more processing power,鈥� says Str枚ckens. But it is unclear why crows evolved to have so many interneurons, he says. No major studies analysing the cognitive abilities of ostriches have been conducted so far, he adds.

High interneuron numbers also aren鈥檛 enough to explain why crows have stronger cognitive abilities than most birds, says Str枚ckens. 鈥淥ther factors like the connectivity between neurons and receptor architecture also play a role.鈥�

鈥淚t is an interesting and important study,鈥� says at Charles University in Prague, Czech Republic. 鈥淭he next frontier for research is to obtain such quantitative data for more species and test which neural feature correlates best with cognitive abilities.鈥�

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