Carrion crow

Name and taxonomy

Carl Linnaeus described the carrion crow in 1758, and the name he gave it still stands. Corvus is the Latin word for a raven, and korone is the Greek word for a crow. The hooded crow was once treated as a subspecies of this bird. Most authorities now list it as the separate species Corvus cornix. The eastern form, C. c. orientalis, may deserve the same treatment.

The two forms of carrion crow sit at opposite ends of the Eurasian landmass. Both hold wet coastal country, and the dry middle of the continent separates them. They may have evolved apart in those two places without ever meeting. Japanese birds hold the mountains, the woods and the cities of that country.

Evolution and relatives

The carrion crow and the hooded crow are as close as two species can be. Researchers sequenced almost the whole genome of both birds. The two proved genetically identical, apart from a patch of under 0.28 per cent. That patch sits on chromosome 18 and controls the pale body plumage of the hooded crow. The glaciations of the Pleistocene probably split one ancestral population into two.

The two birds meet along a narrow contact zone and can breed together. They rarely do, because black birds pair with black birds and grey with grey. Appearance alone appears to keep the two species apart. Researchers still argue about why the preference is so strong. Across Eurasia the bird fills the role that the American crow fills in North America.

Appearance

An adult carrion crow is black from bill to tail. The gloss is green or purple, and it is greener than the sheen on a rook. ADW gives an average length of 47 cm for the species. A wingspan of 84 to 100 cm carries a body of 300 to 600 grams. The bill is heavy and deep, and the feet carry three toes forward and one back.

Young birds are browner than their parents and have blue eyes. Both darken as the bird grows, so an adult shows a brown eye and black feathers. Males and females look the same. The common raven is much bigger, at an average of 63 cm, with a heavier bill. A young rook looks close, but the crow has the larger beak and no bare face patch.

Habitat and range

In Europe the range runs from northern Sweden to southern Spain. The Atlantic is the western edge. In Asia the southern edge crosses Iraq, Iran, Afghanistan and northern China. The birds hold the whole of Japan and the eastern coast of Russia. Breeding reaches about 70 degrees north in Russia, just past the Arctic Circle. A small African population lives along the Nile in Egypt.

Carrion crows once kept to marshland, light farmland and the coast. They have since moved into suburbs and city centres in large numbers. Parks and buildings serve for nesting, and landfill and bins serve for food. City birds find as much food as country birds do. Their measured nutrition is poorer, which is the one clear cost of town life. GBIF holds records of the species from 88 countries.

Diet and feeding

The carrion crow eats almost anything it can handle. Dead animals of every kind form part of the diet, as the name says. It also takes insects, earthworms, grain, fruit, seeds, nuts and small mammals. Amphibians, fish, human scraps and stolen eggs go the same way. Crows harass birds of prey and even foxes until they drop a kill. They sometimes hunt together, and a pair can take a duckling.

Foraging tricks are the part of the diet that draws most study. Birds pull baited fishing lines in with the bill and the feet to steal the catch. Others drop nuts on a road and let the traffic crack them open. Crows hide surplus food when rivals are about. They recall hundreds or thousands of hiding places and what lies in each. They eat perishable stores first and leave the lasting ones for later.

Behaviour and social life

Young carrion crows live in flocks, and a flock of crows is called a murder. A murder holds five to twenty birds from a few families. The group roams a wide area and uses its numbers to win food from rivals. A bird leaves the flock when it finds a mate. The new pair claims a territory and holds it through the year. A defended patch measures 200 to 500 square metres.

Crows mob any predator that enters the territory. A group will dive at a goshawk or an owl until the bird leaves. This works even when the raptor makes no move towards them. Pairs in cold regions may move south for the winter. Winter roosts can be sociable, even for birds that feed alone by day.

The species ranks with the cleverest birds tested. Captive crows tell numbers apart up to thirty. They switch between rules in a task without retraining. They recognise the faces of people and of other crows. A crow brain is built quite differently from a primate brain. The two lines probably reached similar minds by separate routes.

Communication and senses

The call is a deep, slightly grating kraa. A rook gives a higher kaaa, so the voice is the surest way to tell them apart. A crow calls three or four times in quick succession from a roof or a treetop. It pauses between each series and then starts again. With each caw the bird lifts its shoulders and bows its head forward. Captive birds copy sounds, and some copy human speech.

Vision is the strongest sense. In tests the birds followed a hidden object through the moves of a shell game. They found objects from visual clues alone, and they saw through deceptive hiding. Smell matters more than people once assumed. Crows react more to familiar odours than to strange ones. Scent appears to help them find food, avoid predators and know their own kin.

Reproduction and life cycle

Carrion crows form monogamous pairs that stay together for life. Breeding starts in early spring, from March into April. The nest is a bulky cup of sticks, usually high in a tall tree. Cliff ledges, old buildings and pylons serve as well, and a few nests sit near the ground. Both birds build, and the nest is smaller than a raven's.

A clutch holds three to five eggs, blue or greenish with brown speckles. The female alone incubates for 18 to 20 days while the male feeds her. Both parents then feed and guard the chicks. The young leave the nest at about 29 to 30 days old. Families often feed together after the young can fly.

Offspring from the year before sometimes stay to help raise the next brood. About 3 per cent of birds breed co-operatively, and helpers are usually relatives. In parts of northern Spain most nests have helpers, and a group can reach fifteen birds. Both sexes start breeding at about three years old. Roughly half of all crows die before their first birthday. A survivor lives around 10 years, and a captive bird reached 29.

Predators and threats

Few animals take a healthy adult carrion crow. Eurasian goshawks, peregrine falcons, Eurasian eagle-owls and golden eagles all kill them. Crows form a large share of the prey of some raptors in some places. Nobody has studied the predators of this crow directly. Like other corvids, it probably loses eggs and chicks to owls and to mammals.

The great spotted cuckoo lays its eggs in carrion crow nests. Several roundworms live in the bird as parasites. Mobbing is the usual defence, and it almost always sends the attacker away. People remain the largest source of deaths. Farmers and gamekeepers shoot and trap crows over supposed damage to livestock. Research suggests the public greatly overstates that damage.

Population and conservation

The IUCN lists the carrion crow as Least Concern. The wild population is increasing, and no conservation programme is in place. National lists in Finland, Portugal, France and Israel agree with the global one. iNaturalist holds 118,592 observations of the species. The move into towns has probably added to the total.

Persecution continues in farmland across much of Europe. Control appears to be legal in many countries under general pest licences. It has not dented the population at a national scale. The bird probably gains from waste, from roadkill and from open bins. Its future appears safer than that of most farmland birds.

Relationship with humans

Carrion crows live beside people and often lose their fear of them. City birds compete with gulls, ducks and other corvids for scraps in parks. They learn individual people who live near their territory. In one study captive crows scored better with a tester they already knew. That memory for a face may last for years.

Farmers accuse the bird of taking crops and killing young livestock. It does steal grain, and maize is the crop most often named. Reports of lambs killed are far commoner than the evidence supports. Any effect on a national farm economy is probably small. Crows also eat insect larvae and pest species, though nobody has measured the benefit. The species appears in no pet trade and has no market value.

In culture and myth

Crows appear in more European stories than almost any other bird. In Irish tradition the war goddess Badb takes crow shape and is called the battle crow. She spreads fear through an army, and a battlefield could be named her garden. With Macha and the Morrigan she forms a trio of war goddesses. The link between crows and battle probably began with birds feeding on the dead.

In Greek myth Apollo kept a white crow that told him of his lover's betrayal. He scorched its feathers black in his anger, and crows have been black since. A second story has the crow sent for water, delayed by figs and caught in a lie. Apollo threw the bird, its cup and a serpent into the sky as the constellation Corvus. He left the cup beyond reach, so the crow is always thirsty.

In Japanese belief the three-legged crow Yatagarasu guided the first emperor to Yamato. Its name reads as the eight-span crow, a comment on its size. The three legs stand for heaven, earth and humanity in one reading. Another reading gives them as sunrise, daylight and sunset. Modern Japan still uses the bird as a badge, and an asteroid carries its name.