Corvus is a widely distributed genus of birds in the family Corvidae. Ranging in size from the relatively small pigeon-sized jackdaws (Eurasian and Daurian) to the common raven of the Holarctic region and thick-billed raven of the highlands of Ethiopia, the 40 or so members of this genus occur on all temperate continents except South America, and several islands. In Europe, the word “crow” is used to refer to the carrion crow or the hooded crow, while in North America, it is used for the American crow or the northwestern crow.
Corvus species are all black or black with little white or grey plumage. They are stout with strong bills and legs. Sexual dimorphism is limited.
The crow genus makes up a third of the species in the family Corvidae. The members appear to have evolved in Asia from the corvid stock, which had evolved in Australia. The collective name for a group of crows is a ‘flock’ or a ‘murder’.
Recent research has found some crow species capable of not only tool use, but also tool construction. Crows are now considered to be among the world’s most intelligent animals with an encephalization quotient equal to that of many non-human primates.
In medieval times, crow were thought to live abnormally long lives. They were also thought to be monogamous throughout their long lives. They were thought to predict the future, to predict rain and reveal ambushes. Crows were also thought to lead flocks of storks while they crossed the sea to Asia.
Hooded crow (Corvus cornix) in flight
Evolutionary history and systematics
Crow on a branch
The members of the Corvus genus are believed to have evolved in central Asia and radiated out into North America, Africa, Europe, and Australia.
The latest evidence regarding the evolution indicates descent within the Australasian family Corvidae. However, the branch that would produce the modern groups such as jays, magpies, and large, predominantly black Corvus species had left Australasia and were concentrated in Asia by the time the Corvus species evolved. Corvus has since re-entered Australia (relatively recently) and produced five species with one recognized subspecies.
The genus was originally described by Linnaeus in his 18th-century work Systema Naturae. The name is derived from the Latin corvus meaning “raven”.
The type species is the common raven (Corvus corax); others named in the same work include the carrion crow (C. corone), the hooded crow (C. cornix), the rook (C. frugilegus), and the jackdaw (C. monedula). The genus was originally broader, as the magpie was designated C. pica before being moved later into a genus of its own. At least 42 extant species are now considered to be in this genus, and at least 14 extinct species have been described.
No good systematic approach to the genus exists at present. In general, the species from a geographical area are assumed to be more closely related to each other than to other lineages, but this is not necessarily correct. For example, while members of the carrion/collared/house crow complex are certainly closely related, the situation is not at all clear regarding the Australian/Melanesian species. Furthermore, as many species are similar in appearance, determining actual range and characteristics can be very difficult, such as in Australia where the five (possibly six) species are almost identical in appearance.
Close-up of the upper body of a jackdaw (Corvus monedula)
The fossil record of crows is rather dense in Europe, but the relationships among most prehistoric species are not clear.
North American urban corvid history
Corvids are found in major cities across the world, and a major increase in the number of crows in urban settings has occurred since the 1900s. Historical records suggest that the population of American crows found in North America has been growing steadily since the introduction of European colonization, and spread east to west with the opening of the frontier. Crows were uncommon in the Pacific Northwest in the 1900s, except in riparian habitats. Populations in the west increased substantially from the late 1800s to mid 1900s. Crows and ravens spread along with agriculture and urbanization into the western part of North America.
Crows gather in large communal roosts numbering between 200 and tens of thousands of individuals during nonbreeding months, particularly in the winter. These gatherings tend to happen near large food sources such as garbage dumps and shopping centers.
Countless incidents are recorded of corvids at play. Many behaviourists see play as an essential quality in intelligent animals.
Crows and the other members of the genus make a wide variety of calls or vocalizations. Crows have also been observed to respond to calls of other species; presumably, this behavior is learned because it varies regionally. Crows’ vocalizations are complex and poorly understood. Some of the many vocalizations that crows make are a “koww”, usually echoed back and forth between birds, a series of “kowws” in discrete units, a long caw followed by a series of short caws (usually made when a bird takes off from a perch), an echo-like “eh-aw” sound, and more. These vocalizations vary by species, and within each species they vary regionally. In many species, the pattern and number of the numerous vocalizations have been observed to change in response to events in the surroundings (e.g. arrival or departure of crows).
Indian crow in Tamil Nadu
As a group, crows show remarkable examples of intelligence. Natural history books from the 18th century recount an often-repeated, but unproven anecdote of “counting crows” — specifically a crow whose ability to count to five (or four in some versions) is established through a logic trap set by a farmer. Crows and ravens often score very highly on intelligence tests. Certain species top the avian IQ scale. Wild hooded crows in Israel have learned to use bread crumbs for bait-fishing. Crows engage in a kind of mid-air jousting, or air-“chicken” to establish pecking order. They have been found to engage in activities such as sports, tool use, the ability to hide and store food across seasons, episodic-like memory, and the ability to use individual experience in predicting the behavior of environmental conspecifics.
One species, the New Caledonian crow, has also been intensively studied recently because of its ability to manufacture and use tools in the day-to-day search for food. On 5 October 2007, researchers from the University of Oxford presented data acquired by mounting tiny video cameras on the tails of New Caledonian crows. They pluck, smooth, and bend twigs and grass stems to procure a variety of foodstuffs. Crows in Queensland have learned how to eat the toxic cane toad by flipping the cane toad on its back and violently stabbing the throat where the skin is thinner, allowing the crow to access the nontoxic innards; their long beaks ensure that all of the innards can be removed.
The jackdaw and the European magpie have been found to have a nidopallium about the same relative size as the functionally equivalent neocortex in chimpanzees and humans, and significantly larger than is found in the gibbon.
Crows have demonstrated the ability to distinguish individual humans by recognizing facial features. Evidence also suggests they are one of the few nonhuman animals capable of displacement (communicating about things that are happening in a different spatial or temporal location from the here and now).
Corvus splendens or house crow resting in shadows on a rooftop with slaughterhouse refuse to eat
Crows are omnivorous, and their diet is very diverse. They will eat almost anything, including other birds, fruits, nuts, mollusks, earthworms, seeds, frogs, eggs, nestlings, mice, and carrion. The origin of placing scarecrows in grain fields resulted from the crow’s incessant damaging and scavenging, although crows assist farmers by eating insects otherwise attracted to their crops.
Nestlings, almost ready to fledge
Crows reach sexual maturity around the age of 3 years for females and 5 years for males. Clutch size is approximately three to 9 eggs, and the nesting period lasts between 20 and 40 days. Crows often mate for life, and young from previous years often help nesting pairs protect a nest and feed nestlings.
Crow nestlings in urban areas face threats such as nest entanglement from anthropogenic nesting materials and stunted growth due to poor nutrition.
Lifespan and disease
Some crows may live to the age of 20, and the oldest known American crow in the wild was almost 30 years old. The oldest captive crow documented died at age 59.
The American crow is highly susceptible to the recently introduced North American strain of West Nile virus. American crows typically die within one week of acquiring the disease and very few survive exposure.
In the United States, hunting is allowed under state and federal regulation. Crow hunting is considered a sport in rural areas of the U.S. because the birds are not considered a tasty traditional game species. Some cultures do treat various corvid species as a food source. Liability and possible danger to persons and property limit the use of hunting or shooting as control methods in urban areas. Crows’ wariness and cunning make harvesting crows in sufficient numbers difficult.
The Hawaiian crow or ʻalala (Corvus hawaiiensis) is nearly extinct; only a few dozen birds survive in captivity. It is listed as “extinct in the wild” by the United States Fish and Wildlife Service.
Poisoning has been used in the past as a method of controlling crow populations, usually in urban areas or areas of large crow populations. Liability, costs associated with the chemicals, permits and manpower, threats to non target species such as raptors, and availability of chemicals make poisoning a less than desirable method for control.
Scare tactics have been the most widely used aversion tactic for crows in areas frequented by humans and domestic animal species. This safe method does not require constant maintenance or manpower to operate or monitor. However, corvids quickly become habituated to most tactics such as blast cannons, predator decoys, and traditional scarecrows. Greater success has been achieved by adding sound and motion to predator decoys to mimic a distressed crow being caught by a predator such as an owl or hawk. Work is currently being done which uses multiple aversion techniques in one area. The theory is that multiple techniques used together will confuse the crows, thereby lessening the probability of habituation to stimuli.
Trapping is a rarely used technique in the U.S., but is being used with success in parts of Europe and Australia. The ladder-style trap (e.g., Australian Crow Trap or Modified Australian Crow Trap) seems to be the most effective in crow-trapping techniques. Ladder traps are constructed in such a way that unintentional catch of nontarget species is avoided. If a nontarget species is caught, it can be easily released without harm to the bird. The traps are cost-efficient because they are inexpensive and simple to construct, and require little manpower to monitor.
The bait used in the traps can also be specific to corvids. Carrion, grains, unshelled raw peanuts, and shiny objects in the trap are effective baits. When removing crows from a ladder trap, one living crow is left as an extremely effective decoy for other crows. Trapping is considered the most humane method for crow removal because the crows can be relocated without harm or stress. However, most wild birds in general have a knack for returning to their home ranges.
Other methods have been used with little or limited success. Lasers have been used successfully to remove large flocks of birds from roost structures in urban areas, but success in keeping crows off roosts has been short-lived. Homeowners can reduce the presence of crows by keeping trash stored in containers, feeding pets indoors, and hanging tin pie-pans or reflective gazing globes around garden areas.
The common raven and carrion crow have been blamed for killing weak lambs and are often seen eating freshly dead corpses probably killed by other means. The Australian raven has been documented chasing, attacking, and seriously injuring lambs. Rooks have been blamed for eating grain in the UK and brown-necked ravens for raiding date crops in desert countries.
In Auburn, New York, 25,000 to 50,000 American crows (C. brachyrhynchos) have taken to roosting in the small city’s large trees during winter since around 1993. In 2003, a controversial, organized crow hunt proved ineffective at reducing their numbers and the problem (concerns for public health and the sheer noise of so many crows) continues.
At a Technology Entertainment Design conference in March 2008, Joshua Klein presented the potential use of a vending machine for crows. He suggested the crows could be trained to pick up waste and the vending machine would be designed to give a reward in exchange for the garbage.
Crows have been shown to have the ability to visually recognize individual humans and to transmit information about “bad” humans by squawking.
Crows interact with humans and appear to show appreciation by presenting them with gifts.
Cultural depictions and folklore
In folklore, myth, and spirituality
The Twa Corbies by Arthur Rackham
In Ancient Greece and Rome, several myths about crows and jackdaws included:
• An ancient Greek and Roman adage, told by Erasmus runs, “The swans will sing when the jackdaws are silent,” meaning that educated or wise people will speak after the foolish become quiet.
• The Roman poet Ovid saw the crow as a harbinger of rain (Amores 2,6, 34).
• Pliny noted how the Thessalians, Illyrians, and Lemnians cherished jackdaws for destroying grasshoppers’ eggs. The Veneti are fabled to have bribed the jackdaws to spare their crops.
• Ancient Greek authors tell how a jackdaw (a passerine bird in the crow family), being a social creature, may be caught with a dish of oil into which it falls while looking at its own reflection.
• In Greek legend, princess Arne was bribed with gold by King Minos of Crete and was punished for her avarice by being transformed into an equally avaricious jackdaw, which still seeks shiny things.
In Australian Aboriginal mythology, Crow is a trickster, culture hero, and ancestral being. Legends relating to Crow have been observed in various Aboriginal language groups and cultures across Australia; these commonly include stories relating to Crow’s role in the theft of fire, the origin of death, and the killing of Eagle’s son.
Crows are mentioned often in Buddhism, especially Tibetan disciplines. The Dharmapala (protector of the Dharma) Mahakala is represented by a crow in one of his physical/earthly forms.
House crow (Corvus splendens), Bangalore
In the Chaldean myth, the Epic of Gilgamesh, Utnapishtim releases a dove and raven to find land; however, the dove merely circles and returns. Only then does Utnapishtim send forth the raven, which does not return, and Utnapishtim concludes the raven has found land.
In Chinese mythology, the world originally had 10 suns either spiritually embodied as 10 crows and/or carried by 10 crows; when all 10 decided to rise at once, the effect was devastating to crops, so the gods sent their greatest archer Houyi, who shot down nine crows and spared only one.
In Denmark, the night raven is considered an exorcised spirit. A hole in its left wing denotes where the stake used to exorcise it was driven into the earth. He who looks through the hole will become a night raven himself.
Crow on a Branch – Kawanabe Kyosai (1831–1889)
In Hinduism, crows are thought of as carriers of information that give omens to people regarding their situations. For example, when a crow crows in front of a person’s house, the resident is expected to have special visitors that day. Also, in Hindu literature, crows have great memories which they use to give information. Symbolism is associated with the crow in the Hindu faith.
On a positive note, crows are often associated with worship of ancestors because they are believed to be embodying the souls of the recently deceased. However, many negative associations with crows are seen in Hinduism. Crows are believed to be connected with both the gods and goddesses, particularly the unfavorable or harmful ones such as Sani. They are often seen as dark and dangerous. Crows are also seen as being signs of bad luck or evil in some practices. Crows are also considered ancestors in Hinduism and during Śrāddha, the practice of offering food or pinda to crows is still in vogue.
In Irish mythology, crows are associated with Morrigan, the goddess of war and death.
In Islam, according to a narration in the Hadith, the crow is one of the five animals for which no blame is placed on the one who kills them. The Surat Al-Ma’ida describes the story of how the crow teaches son of Adam to cover dead body of his brother “Then Allah sent a crow digging up the earth so that he might show him how he should cover the dead body of his brother. He said: Woe me! do I lack the strength that I should be like this crow and cover the dead body of my brother? So he became of those who regret.”.
In Japanese mythology, a three-legged crow called Yatagarasu (八咫烏?, “eight-hand-crow”) is depicted.
Crow on a branch,Maruyama Ōkyo (1733–1795)
In Korean mythology, a three-legged crow is known as Samjokgo (hangul: 삼족오; hanja: 三足烏).
In Norse mythology, Huginn and Muninn are a pair of common ravens that range the entire world, Midgard, bringing the god Odin information.
In Sweden, ravens are held to be the ghosts of murdered men.
In Welsh mythology, the god Brân the Blessed – whose name means “crow” or “raven” — is associated with corvids and death; tradition holds that Bran’s severed head is buried under the Tower of London, facing France — a possible genesis for the practice of keeping ravens in the Tower, said to protect the fortunes of Britain. In Cornish folklore, crows — magpies particularly — are associated with death and the “other world”, and proscribes respectful greeting. The origin of “counting crows” as augury is British; however, the British version rather is to “count magpies” — their black and white pied colouring alluding to the realms of the living and dead.
Jungle crow (Corvus macrorhynchos) scavenging on a dead shark at a beach in Kumamoto, Japan