Common raven

• 1. Introduction

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  A piece of meat hangs from a perch on a length of string. Five common ravens were given this puzzle in an experiment on insight and problem-solving. Four of them solved it. To do so, a bird had to stand on the perch, pull the string up a little at a time, and step on each loop to hold it. The researchers reported that the shift from ignoring the food or merely yanking at the string to reliable success came with no demonstrable trial-and-error learning. The bird behind that result is Corvus corax, the common raven, also called the northern raven. It is a large, all-black passerine, the heaviest bird in that perching group. People have lived alongside it for thousands of years. Where did this bird come from, and what does it look and sound like? How does it raise a family and feed itself? And why have so many cultures treated it as something more than an animal?

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• 2. Origins and Range

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  In 1758, Carl Linnaeus described the common raven in the tenth edition of his Systema Naturae, giving Europe as the place the specimen came from. The bird still carries the name he chose, Corvus corax, and it is the type species of the genus Corvus. Corvus is simply the Latin word for raven. Corax is the Latinized form of the Greek word κόραξ, which meant raven or crow. The English word has cousins across the Germanic languages, including Old Norse hrafn and Old High German (h)raban, all descended from Proto-Germanic hrabanaz. Scots once used corby or corbie, akin to the French corbeau, for both this bird and the carrion crow. A group of ravens can be called an unkindness or a conspiracy.

  DNA collected from common ravens around the world sorts them into at least two clades, or family branches. One is the California clade, tied to the subspecies C. c. clarionensis and found only in the southwestern United States. The other is the Holarctic clade, which covers the rest of the Northern Hemisphere. Birds from the two look alike, yet they began to diverge about two million years ago. Mitochondrial DNA shows that the California ravens are closer kin to the Chihuahuan raven than to other common ravens. Holarctic ravens, meanwhile, are closer to the pied crow of Africa. That tangle leaves the species, as traditionally drawn, paraphyletic.

  One explanation starts in the Old World, where the raven evolved before crossing the Bering land bridge into North America. Ravens may have settled in California at least as long ago as the split, then been cut off from their Eurasian relatives during a glacial period. About a million years ago, a group from that California line became the Chihuahuan raven. The Holarctic ravens would have arrived later in a separate migration from Asia, perhaps about 15,000 years ago, around the time humans and wolves made the same crossing. A 2011 study found no restriction on gene flow between the two groups, so the lineages could merge again and undo a possible speciation.

  On Mount Everest, a common raven has been recorded as high as 6,350 m, or 20,600 ft. Few birds match the range. It stretches from Arctic and temperate North America and Eurasia to the deserts of North Africa, and out to islands in the Pacific. Most authorities, including the IOC World Bird List and the Handbook of the Birds of the World, accept 11 subspecies, though some accept only eight. They run from the largest and glossiest, C. c. tibetanus of the Himalaya at 2,500-5,000 m, to the smallest, C. c. tingitanus of North Africa. Ravens are generally resident all year, and in Arctic Canada and Alaska they stay through winter, although Finn Salomonsen's 1950 work Grønlands Fugle said they did not overwinter in the Arctic.

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• 3. Body and Voice

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  Ravens from California average 0.784 kg, those from Alaska 1.135 kg, and those from Nova Scotia 1.230 kg. Colder regions such as the Himalayas and Greenland produce larger birds with slightly larger bills, while warmer regions produce smaller birds with proportionally smaller bills. Across the species, a mature raven measures between 54 and 71 cm, averaging 63 cm, with a wingspan of 116 to 153 cm. Recorded weights run from 0.69 kg to 2.250 kg, with an average of 1.47 kg.

  The bill is large and slightly curved, with a culmen length of 5.7 to 8.5 cm. Only the thick-billed raven and the white-necked raven have bigger bills among passerines. The tail is long and wedge-shaped, at 20 to 26.3 cm, and the plumage is mostly iridescent black. Adults have a dark brown iris, while juveniles are duller, with a blue-grey iris and a pinkish gape at first. The throat feathers are elongated and pointed, giving the bird a shaggy beard.

  A flying raven is easy to tell from a crow. The raven has a wedge-shaped tail, a larger wing area, and a steadier soaring style with less flapping. Despite its bulk, it is easily as agile as its smaller cousins. In flight its feathers make a creaking sound that has been likened to the rustle of silk. The Faroe Islands once held a pied colour morph, now extinct. White ravens turn up occasionally in the wild, and some in British Columbia lacked the pink eyes of an albino, being leucistic instead.

  In the early 1960s, a researcher named Gwinner recorded and photographed raven behaviour in great detail, and his work became central to the study of their calls. Between 15 and 30 categories of call have since been recorded, most of them used for social interaction. They include alarm calls, chase calls, and flight calls. The signature sound is a deep, resonant prruk-prruk-prruk, unlike that of any other corvid to an experienced listener. The vocabulary also holds a high, knocking toc-toc-toc, a dry, grating kraa, a low guttural rattle, and some calls that are almost musical.

  Ravens mimic sounds from their surroundings, including human speech. They also make non-vocal sounds, such as wing whistles and bill snapping. Clapping or clicking has been heard more often from females than from males. When a pair is broken by a loss, the surviving mate reproduces the calls of its lost partner to encourage its return.

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• 4. Family Life

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  Young ravens begin to court at a very early age, yet they may not bond for another two or three years. Courtship leans on aerial acrobatics, demonstrations of intelligence, and the ability to provide food. Once paired, they tend to nest together for life, usually in the same place. Males have been seen visiting a female's nest while her mate is away, so the bond is not perfectly exclusive. Relationships among ravens are often quarrelsome, yet the birds show considerable devotion to their families.

  A pair needs a territory of its own before it builds a nest, and it defends that ground and its food fiercely. Territory size depends on how thick the food supply is. The nest is a deep bowl of large sticks, some up to 150 cm long and 2.5 cm thick. An inner layer of roots, mud, and bark binds it, and a softer lining such as deer fur finishes it. It usually sits in a large tree or on a cliff ledge, and less often in old buildings or on utility poles.

  The female lays four to six eggs, rarely two to seven, pale bluish-green with brown blotches. She alone incubates them for about 18 to 21 days. The male may stand or crouch over the young, sheltering them without actually brooding. The chicks fledge at 35 to 49 days, both parents feed them, and they stay with their parents for another six months. Timing shifts with climate. Egg-laying usually begins in late January or February, but it can wait until April in Greenland and Tibet. In Pakistan it happens in December, and in North Africa it comes in late March or early April.

  Martens, large owls, and sometimes eagles take eggs and chicks. Adult ravens attack such threats by flying at them and lunging with their big bills, and they usually drive them off. They have even been seen dropping stones on predators that come near the nest. Adults are very rarely preyed upon. In North America the reported predators include great horned owls, American goshawks, bald eagles, golden eagles, and red-tailed hawks. In Eurasia they include Eurasian eagle-owls, white-tailed eagles, Steller's sea-eagles, eastern imperial eagles, and gyrfalcons. Because a raven is hazardous prey, raptors usually have to surprise it, and most attacks fall on fledglings.

  Individuals at the Tower of London have lived for more than 40 years. Wild ravens typically live 10 to 15 years. The longest known wild lifespan, for a ringed bird, was 23 years and 3 months, a mark that among passerines only a few Australian species, such as the satin bowerbird, surpass.

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• 5. Eating Everything

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  On the Arctic North Slope of Alaska, ravens foraging on the tundra got about half their energy from hunting, mainly microtine rodents, and half from scavenging, mainly caribou and ptarmigan carcasses. Their diet shifts with location, season, and luck. They eat carrion and the maggots and carrion beetles on it, cereal grains, acorns, buds, berries, and fruit. They also take invertebrates, amphibians, reptiles, small mammals, and birds, along with the afterbirth of ewes and other large mammals. Human food waste and even the undigested parts of animal faeces are on the menu.

  A large carcass poses a problem. Ravens cannot tear through it as well as the much larger, hook-billed vultures, so they must wait until another predator opens it or it is flayed by other means. Wolves often fill that role. Ravens follow wolves as kleptoparasites, scavenging wolf-kills in winter. They have also been observed leading hunting packs to prey that only the ravens could see from the air.

  Surplus food, especially anything fatty, gets stored. Ravens learn to hide it out of sight of other ravens, and they raid the caches of other species, such as the Arctic fox. They are wary around novel carrion, and in North America they have been recorded waiting for American crows and blue jays to appear before they approach to eat.

  The California condor shows the raven at its most brazen. Ravens readily take condor eggs and are very common where the condor is being re-introduced, so they are considered perhaps the primary natural threat to its nesting success. They also pick off the eggs and nestlings of other birds, and sometimes adults. Yet when they defend their own nearby nests, they chase golden eagles away, which may incidentally protect larger condor nestlings. Condors recognise ravens as threats and chase them off. Even so, their nest sites are poorly concealed from ravens, for reasons nobody knows.

  Human waste changes the menu. Ravens nesting near garbage ate more food waste, those near roads ate more road-killed vertebrates, and those far from either ate more arthropods and plants. Fledging success was higher for those using garbage. A 1984-1986 study in an agricultural region of southwestern Idaho found cereal grains were the main constituent of pellets, alongside small mammals, grasshoppers, cattle carrion, and birds. Some research also suggests that ravens disperse seeds, choosing locations well suited to survival.

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• 6. Mind and Play

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  The linguist Derek Bickerton, building on the work of biologist Bernd Heinrich, argues that ravens are one of only four known animals to show displacement. The others are bees, ants, and humans. Displacement is the ability to communicate about objects or events far away in space or time. Subadult ravens roost together at night but usually forage alone by day. When one finds a large carcass guarded by a pair of adults, it returns to the roost and passes on the news. The next day a flock of unmated birds flies to the carcass and chases the adults off. Bickerton argues that linguistic displacement may have been the most important event in the evolution of human language.

  Heinrich, in Ravens in Winter, proposed that juvenile ravens call others to a carcass with loud yells so they can outnumber the resident adults. A plainer explanation is that birds share news of large carcasses because the meal is too big for a few to handle. Experiments with baits, however, show that the calling does not depend on bait size.

  A study published in 2011 found that ravens recognise an unfair trade in dealings with other ravens or with humans, and they remember it for a long time. Birds given a fair trade preferred the experimenters who had treated them fairly. In the wild, ravens have been seen to stop cooperating with others they catch cheating during group tasks. Ravens also watch where others bury food and remember the locations so they can steal from them. The theft is so regular that they fly extra distances to find better hiding places. They have been seen pretending to make a cache without depositing anything, presumably to confuse onlookers. Another experiment showed that some ravens can deliberately deceive their fellows.

  Golf balls, pebbles, and pieces of metal all end up in raven caches. One theory holds that they hoard shiny objects to impress other ravens. Other research indicates that juveniles are deeply curious about anything new, and that ravens keep an attraction to bright, round objects because they resemble eggs. Mature birds lose that intense interest in the unusual and become highly neophobic. Underneath it all sits one of the largest brains of any bird, with an unusually large hyperpallium. By four months of age, ravens do about as well as adult chimpanzees and orangutans on tests of causal reasoning, social learning, and theory of mind.

  Juvenile ravens are among the most playful of birds. They slide down snowbanks apparently purely for fun. They play catch-me-if-you-can with wolves, otters, and dogs. Adults put on spectacular aerobatic displays, flying in loops or interlocking talons in mid-air, and they often soar over hilly terrain to take advantage of ridge lift. Ravens are also one of only a few wild animals that make their own toys, breaking off twigs to play with socially.

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• 7. Ravens and People

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  In the western Mojave Desert, human settlement and land development have driven an estimated 16-fold increase in the common raven population over 25 years. Towns, landfills, sewage treatment plants, and artificial ponds provide food and water. Utility poles and ornamental trees offer nesting sites, and highways supply roadkill. The surge has raised concerns for the threatened desert tortoise, because ravens prey on juvenile tortoises, which have soft shells and move slowly. Plans to control the birds have included shooting and trapping them, and asking landfill operators to reduce exposed garbage. In Alaska, limited culling has taken place because the growing raven population threatens the vulnerable Steller's eider. California, too, has seen a surge in numbers where people supply plentiful food.

  Finland paid a hunting bounty on ravens from the mid-18th century until 1923. Farmers elsewhere have their own complaints. Ravens can damage crops such as nuts and grain, and they can harm livestock, particularly by killing young goat kids, lambs, and calves, usually attacking the face. Yet their ordinary scavenging may be misidentified as predation by ranchers.

  Not every region has too many. Compared with smaller relatives such as the carrion crow and American crow, ravens prefer undisturbed mountain, forest, or rural habitat over cities. In the British Isles they are most common in Scotland, Wales, western England, and Ireland, having been eradicated from other areas by gamekeeping interests. In the United Kingdom their range is now growing after improved legal protection, with expansion southwards, though illegal persecution by gamekeepers remains a problem in many areas.

  Ravens, like other corvids, are definitive hosts of West Nile Virus. Culex mosquitoes can carry it from infected birds to humans, and ravens are susceptible. A 2010 study, however, showed that California common ravens did not have a high positivity rate.

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• 8. Myth and Symbol

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  In Norse mythology, two ravens named Huginn and Muninn fly over Midgard, the world of humans, and bring the god Odin information. Their names come from the Old Norse for thought and for memory or mind. Norse leaders also marched under raven banners, among them the Jarls of Orkney, King Cnut the Great of England, Norway, and Denmark, and Harald Hardrada. In Irish mythology, the goddess Morrígan alighted on the shoulder of the hero Cú Chulainn in the form of a raven after his death. In Welsh mythology the bird was linked to the god Brân the Blessed, whose name translates to raven and crow.

  According to the Mabinogion, Brân's head was buried in the White Hill of London as a talisman against invasion. A later legend says England would not fall to a foreign invader as long as ravens lived at the Tower of London. Many assume that belief is ancient. The Tower's official historian, Geoff Parnell, believes it is actually a romantic Victorian invention.

  For the Tlingit and Haida, Raven was both trickster and creator god. Related beliefs are widespread among the peoples of Siberia and northeastern Asia. The Kamchatka Peninsula, for example, was said to have been created by the raven god Kutkh. In Bhutanese mythology the raven is an aspect of Mahakala.

  In the Jewish, Christian, and Islamic traditions, the raven was the first animal released from Noah's Ark, sent out after 40 days. The Bible mentions it 12 times. In the New Testament, Jesus uses ravens in a parable, at Luke 12:24, to show that people should rely on God rather than riches. The Quran has a raven teach Cain how to bury his brother Abel.

  Other traditions are darker. In some Western traditions the raven is a bird of ill omen, partly because of its black plumage and its taste for carrion. In Sweden ravens are the ghosts of murdered people, and in Germany they are the souls of the damned. Danish folklore tells of the valravne, which ate a king's heart, gained human knowledge, and became a terrible animal. In Persia and Arabia the raven was a bad omen too, yet a 14th-century Arabic work records its use in falconry.

  The symbol still works in places where the bird once had mythological status. The raven is the official bird of the Yukon territory and appears on the coat of arms of the Isle of Man, once a Viking colony. In Bhutan it is the national bird, and the kings of Bhutan wear the Raven Crown.

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