How Clever Are Crows? Tools, Faces and Planning for Tomorrow

Crows, ravens, rooks, jackdaws, magpies and jays are corvids, a family of about 130 species found on every continent except South America and Antarctica, and they are, with the parrots, the birds that pass the tests designed for apes. New Caledonian crows are the tool specialists, ravens the social strategists, and the western scrub-jay the champion of memory; a raven weighs a kilogram and a New Caledonian crow a quarter of that, and neither has much room for a brain. The reason they manage is density. Bird brains pack neurons far more tightly than mammal brains do, and a 2016 count found that a raven's forebrain holds about as many neurons as a small monkey's, in a fraction of the volume, connected through a structure called the nidopallium that does the work a mammal's cortex does without being arranged like one.

The experimental record, mostly from the last twenty-five years, is long:

• •Tool use and manufacture: New Caledonian crows cut hooked probes from pandanus leaves in a standard shape, carry favourite tools around, and use one tool to obtain another in sequences of up to three, a skill called metatool use that only apes had shown
• •Causal reasoning: crows dropped stones into a water-filled tube to raise a floating reward, chose the stones that would work, and ignored a tube of sawdust, matching the performance of a five-to-seven-year-old child
• •Faces: after being caught and released by researchers in masks, crows in Seattle mobbed anyone wearing that mask for years and taught their offspring and neighbours to do the same, so that the number of birds scolding it grew after the original birds had died
• •Memory for caches: scrub-jays hide thousands of items and recall not only where but what and when, retrieving perishable food first, which is evidence of episodic memory of a kind once thought uniquely human
• •Planning: ravens selected and kept a tool for a task they would be offered a day later, and traded tokens for future rewards, performing as well as apes
• •Deception: a jay that is watched while caching will return alone and move the food, and only if the watcher was another jay

The last common ancestor of a crow and a person was a small reptile about 320 million years ago, and the brains built since have almost nothing structurally in common: mammals have a layered cortex, birds have clustered nuclei, and for a century the bird brain was taken to be a primitive thing incapable of much. The abilities are the same and the hardware is different, which makes corvids the strongest case in biology for convergent evolution of intelligence, the same solution reached twice. It has consequences for the argument about what a mind needs. Whatever it is that supports planning, causal inference and self-control, it is not the mammalian cortex specifically, and a 2020 study that found neurons in a crow's brain responding to what the bird reported seeing rather than to what was shown has been read as evidence of something like conscious perception in an animal that diverged from us before the dinosaurs.

The usual explanation is a combination of pressures. Corvids are long-lived, up to twenty years in the wild and forty in captivity, which repays learning; they live in complex social groups where remembering who cooperated and who cheated pays; they are generalists that eat almost anything and must solve new problems in every season; and the tool-using species live on islands with no woodpeckers and a rich supply of grubs in dead wood, so that a bird able to extract them had a niche to itself. The same list, long life, sociality, generalist diet and extractive foraging, is what is usually given for primates, which is part of why the two families ended up in the same place.

Crows do well near people because rubbish is a generalist's paradise and cities have few predators, and the relationship is close enough that they watch. Individual crows have been recorded bringing small gifts, a bottle top, a bead, a screw, to people who fed them; urban crows in Japan learned to place walnuts in front of cars at traffic lights and to collect the kernels when the lights changed; and the birds that follow a person along a street in the morning may be the same ones every day. They have also been persecuted as crop pests and carriers of ill omen for as long as there are records, which they appear to remember. The advice from the researchers who work on them is straightforward: do not annoy a crow, because it will know you, and so will its children.

Crows and their relatives make hooked tools, use one tool to get another, reason about causes, recognise individual human faces and pass the grudge to their young, remember what they cached and when, plan for tomorrow and deceive rivals who are watching, matching great apes on tests designed for apes. They do it with a walnut-sized brain that packs neurons far more densely than a mammal's and is organised differently, which makes them the clearest evidence that intelligence has evolved more than once and needs no cortex.