How intelligent are crows?

Corvids, the family including crows, ravens, and jays, show cognitive abilities that repeatedly surprise researchers. New Caledonian crows manufacture hooked tools from twigs and use them in sequence, retrieving a short stick to obtain a longer one to reach food, a multi-step planning task that few animals solve. Crows have passed versions of the Aesop's fable test, dropping stones into water to raise the level and reach a floating reward, demonstrating grasp of water displacement. Some studies suggest they can plan for future needs and exercise self-control by declining an immediate reward for a better one later.

The social cognition is equally notable. University of Washington research had volunteers wear distinctive masks while trapping crows, and the birds recognized and scolded those specific masks years afterward, including crows that had never been trapped themselves, meaning the information spread socially through the population and even across generations. Crows hold what functionally amount to grudges and can also recognize and favor individual humans who feed them, sometimes leaving small objects in apparent exchange. Their brains achieve this with an architecture entirely different from mammalian cortex, packing very high neuron densities into a small forebrain, which makes them an important case for understanding that complex intelligence evolved more than once by different routes.