Crow

Crows recognize individual human faces, remember who treated them badly, and tell other crows about it. Young crows that were never present at the original incident will mob a person their parents learned to distrust, years later.

They also make tools, solve multi-step puzzles, hold what look like funerals, and sometimes bring gifts to people who feed them.

On several tests of intelligence crows perform at the level of apes, which is unexpected in an animal whose brain is the size of a walnut.

Crows belong to the genus Corvus, with around 45 species, so there is no single two-word name. The American crow is Corvus brachyrhynchos and the carrion crow is Corvus corone.

• Kingdom: Animalia
• Phylum: Chordata
• Class: Aves
• Order: Passeriformes
• Family: Corvidae
• Genus: Corvus

Worth knowing: ravens are crows. They are in the same genus, simply the largest members of it. The family Corvidae also includes jays, magpies, jackdaws, rooks and nutcrackers, and the whole family is unusually intelligent.

Crows are songbirds, technically. They are in the same order as sparrows and nightingales, which says something about how loosely that term fits.

Remarkably so, and the experiments are worth going through because each one rules out a simpler explanation.

• They know faces. Researchers wearing a particular mask trapped and released crows. For years afterward the birds scolded anyone wearing that mask and ignored other masks. The knowledge spread to crows that had never been trapped at all.
• They make tools. New Caledonian crows strip and shape twigs into hooks to pull grubs from holes, and they make different tool designs in different regions, passing the designs on.
• They make tools from parts. In one experiment crows assembled a long tool out of short pieces that were individually too short to reach the food, with no training.
• They understand water displacement. Offered a tube of water with food floating out of reach, crows dropped stones in to raise the level, choosing solid objects over hollow ones. Children manage this at about five to seven years old.
• They plan ahead. Crows select and keep a tool they will need later, which requires imagining a future situation.

The reason is in the brain. Bird brains pack neurons far more densely than mammal brains, so a crow has roughly as many neurons in its forebrain as a small monkey, in a fraction of the space.

Both are in the genus Corvus, so a raven is a kind of crow rather than a separate thing.

Crows measure 16 to 21 inches long with a wingspan of 33 to 39 inches, and weigh 11 to 22 ounces. Ravens are considerably bigger.

• Feathers. Black, but not plain black. In sunlight they show a purple and blue iridescence.
• Beak. Strong and general-purpose, suited to tearing, probing, cracking and carrying.
• Brain. Large for the body, with densely packed neurons in the forebrain.
• Eyes. Excellent color vision, better than ours, extending into ultraviolet in some species.
• Feet. Strong enough to hold food down while the beak works on it, which many birds cannot do.

Males and females look identical, which is unusual for a bird and means crows must recognize each other by voice and behavior rather than appearance.

Crows live on every continent except South America and Antarctica, in woodland, farmland, coast, mountain and city.

They are among the great urban successes. Cities offer food, warmth, nesting sites and few predators, and crows learn city life fast.

The most famous example comes from Japan, where carrion crows drop walnuts onto pedestrian crossings, wait for cars to crack them, then collect the pieces when the lights stop the traffic. They use the crossing because it is the only moment it is safe to walk out.

Crows are omnivores and eat almost anything: insects, worms, seeds, grain, fruit, eggs, nestlings, small mammals, fish, carrion and human food waste.

That flexibility is the foundation of everything else about them. An animal that can eat anything can live anywhere, and benefits from being clever enough to work out how to get at it.

They cache food, hiding surplus and remembering hundreds of locations. They also watch other crows caching, and will move their own food if they were observed, which means they understand that another bird saw them.

They drop hard-shelled food from height onto rock or road to break it open.

Crows are highly social. Families stay together, and young from previous years often help raise the next brood rather than leaving to breed themselves.

They roost communally in winter, sometimes in gatherings of thousands.

Crow funerals are real and have been studied. When a crow finds a dead crow, others gather, call loudly and stay near the body for a time. They are not mourning as we would: experiments suggest they are learning what killed it and what to avoid, treating the death as information about danger.

They also play, sliding down snowy roofs, hanging upside down from branches and teasing other animals, which is hard to explain as anything but fun.

And they do sometimes bring small objects to people who feed them regularly, though how deliberate the gift-giving is remains debated.

Crows pair for life and build bulky stick nests high in trees.

The female lays 3 to 6 blue-green speckled eggs and incubates them for about 18 days while the male brings food.

Chicks fledge at around five weeks but stay with their parents for a year or more, learning. That long apprenticeship is part of why crows are so capable: they are taught.

Crows live 7 to 8 years on average in the wild, with some reaching 20, and captive birds have passed 30.

Hawks, owls and eagles take crows, and nests are raided by squirrels, snakes and other crows. Great horned owls are a particular enemy, which is why crows mob them relentlessly in daylight.

Crows are shot and poisoned as agricultural pests in many countries, though their actual impact on crops is often smaller than assumed, and they eat large numbers of insect pests.

West Nile virus killed large numbers of American crows in the 2000s, and they remain unusually vulnerable to it.

Most crows are abundant and in no danger, and several species are expanding as cities spread.

A few island species are in serious trouble. The Hawaiian crow went extinct in the wild in 2002 and survives only in captivity, with reintroductions attempted. The Mariana crow is Critically Endangered with very few birds left.

The pattern is familiar: widespread adaptable crows thrive, and island crows with nowhere to go do not.

Crows are among nature’s most effective cleaners. They eat carrion in quantity, including roadkill, removing material that would otherwise rot and spread disease.

They control insects, including agricultural pests, and disperse seeds, particularly from the nuts and acorns they cache and fail to recover. Buried and forgotten seeds grow into trees.

They do take eggs and nestlings of other birds, which can matter locally where a songbird population is already under pressure.

Scientifically they have been enormously valuable. Crows forced a rethink of how intelligence evolves, by showing that a brain built on a completely different plan from ours, with no cortex at all, can produce planning, tool-making and social reasoning. Intelligence turns out to have been invented more than once.