Temporal range: 17–0Ma Middle Miocene – Recent
|American crow (Corvus brachyrhynchos)|
|c. 40 species|
Corvus is a widely distributed genus of birds in the family Corvidae. Ranging in size from the relatively small pigeon-size 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 "raven" 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.
The crow genus makes up a third of the species in the Corvidae family. 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 approaching that of some apes.
In medieval times, the crow was thought to live an abnormally long life. They were also thought to be monogamous throughout their long lives. It was thought that the crow could predict the future, in that it was thought to predict rain and reveal ambushes. Crows were also thought to lead flocks of storks while they crossed the sea to Asia.
Corvus species are all black or black with little white or grey plumage. They are stout with strong bills and legs. There is limited sexual dimorphism.
Evolutionary history and systematics
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 had left Australasia and were concentrated in Asia by the time the Corvus evolved. Corvus has since re-entered Australia (relatively recently) and produced five species with one recognized sub-species.
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. There are now considered to be at least 42 extant species in this genus, and at least 14 extinct species have been described.
There is not a good systematic approach to the genus at present. In general, it is assumed that the species from a geographical area are 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.
The fossil record of crows is rather dense in Europe, but the relationships among most prehistoric species are not clear.
North American urban crow history
Crows are found in major cities across the world, and there has been a major increase in the number of crows in urban settings 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 spread along with agriculture and urbanization into the western part of North America. 
Crows gather in large communal roosts between 200 and tens of thousands of individuals during non-breeding months, particularly in the winter. These gatherings tend to happen near large food sources such as garbage dumps and shopping centers. 
There are countless recorded incidents of crows 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). Crows can imitate calls of other birds. A local crow was observed, for many years now, to "blend in" with pigeons and imitate a pigeon's cooing in order to feed on food fed to pigeons by humans. This crow "pretends" to be a pigeon to get food. Call sounds almost like pigeon cooing but still different from real pigeon's cooing. The same crow made usual (normal) crow calls when it was observed among other crows.
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 will engage in a kind of mid-air jousting, or air-"chicken" to establish pecking order. Crows have been found to engage in feats 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, England, 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, Australia 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 non-toxic 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 approximately 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 that they are one of the few non-human animals capable of displacement (communicating about things that are happening in a different spatial or temporal location to the here and now).
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.
Crows reach sexual maturity around the age of 3 years for females and 5 years for males. Clutch size is approximately 3-9 eggs, and the nesting period lasts between 20 and 40 days. Young from previous years often help nesting pairs protect a nest and feed nestlings. 
Life span and disease
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.
Two species of crow have been listed as endangered by the U.S. Fish and Wildlife Service: the Hawaiian crow and the Mariana crow. The American crow, despite having its population reduced by 45% since 1999 by the West Nile virus, is considered a Species of Least Concern.
Problems with crows and methods of control
Intelligence and social structures makes most crow species, or Corvids, an adaptable and opportunistic species. Crows frequently cause damage to crops and property, strew trash, and transfer disease. In densely populated areas around the world, Corvid species are generally regarded as nuisance animals. Crows are protected in the U.S. under the federal Migratory Bird Treaty Act of 1918, but because of their perceived destructive nature, control of the species is allowed in certain areas. Because of the Corvids' intelligence, their control is an expensive and perplexing proposition. Methods for control include hunting, chemical immobilization, harassment scare tactics, and trapping, as well as others. Before any measure is used to confine, trap, kill, poison, immobilize, or alter the habits of any wild bird species a person must check local, state, and federal regulations pertaining to such actions.
In the United States, despite crows being protected under the Federal Migratory Bird Treaty Act of 1918, 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 it difficult to harvest crows in sufficient numbers.
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, cost associated with the chemicals, permits and manpower, threat to non-target species such as raptors, and availability of chemicals makes poisoning less than a 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 is a safe method that 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 non-target species is avoided. If a non-target 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.
As a food supplement
Crows were hunted for survival by Curonians, a Baltic tribe, when common food was exhausted and the landscape changed so that farming was not as productive during the 18th and 19th centuries. Fishermen supplemented their diet by gathering coastal bird eggs and preserving crow meat by salting and smoking it. It became a traditional food for poor folk and is documented in a poem, The Seasons by K. Donelaitis. After the non-hunting policy was lifted by the Prussian government in 1721–24 and alternative food supplies increased, the practice was forgotten. The tradition reemerged after World War I; in marketplaces, it was common to find butchered crows which were sought after and bought by townsfolk. The hunted crows were not the local, but the migrating ones; each year during the spring and autumn crows migrated via the Curonian Spit between Finland and the rest of Europe. In 1943, the government even issued a hunting quota for such activities. Crows were usually caught by attracting them with smoked fish or grains soaked in spirits and then collecting them with nets. It was a job for the elderly or young who were unable to go to sea to fish, and it was common to catch 150 to 200 birds during a hunting day.
In human culture
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 raven for raiding date crops in desert countries.
In Auburn in the U.S. state of 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.
Myth and spirituality
The god Bran 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 "otherworld", 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.
In Sweden, ravens are held to be the ghosts of murdered men. In Denmark, the night raven is considered an exorcised spirit. There is a hole in its left wing where the stake used to exorcise it was driven into the earth. Those looking through the hole will become a night raven themselves.
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.
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.
According to Ovid's Metamorphoses, in Greek mythology, the god Apollo became enraged when the crow exposed his lover Coronis' tryst with a mortal, his ire transmuting the crow's feathers from white to black.
In Chinese mythology, the world originally had ten suns either spiritually embodied as ten crows and/or carried by ten crows: when all ten 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. This mythology comes from a text in Shanhaijing, among other sources.
In Hinduism, crows are thought of as carriers of information. They give omens to people regarding their situations. For example, when a crow crows in front of a person's house, he is expected to have special visitors that day. Also, in Hindu literature, crows have great memories which they use to give information. There is a lot of symbolism associated with the crow in the Hindu faith. On a positive note, Crows are often associated with worship to ancestors because they are believed to be embodying the souls of the recently deceased. On the other hand, there are a lot of negative associations with crows in Hinduism. Crows are believed to be connected with both the gods and goddesses, particularly the unfavorable or harmful ones like Sani. They are often seen as dark and dangerous. Crows are also seen as being a sign of bad luck or evil in some practices. However, recently their perception has been a topic of discussion, and it has been concluded that it is unclear.
In the Story of Bhusunda, a chapter of the Yoga Vasistha, a very old sage in the form of a crow, Bhusunda, recalls a succession of epochs in the earth's history, as described in Hindu cosmology. He survived several destructions, living on a wish-fulfilling tree on Mount Meru. Crows are also considered ancestors in Hinduism and during Śrāddha, the practice of offering food or pinda to crows is still in vogue.
In Islam, according to a narration in the Hadith, the crow is one of the five animals for which there is no blame on the one who kills them. In Surat Al-Ma'ida 5:31, Allah sent a crow to one of the sons of Adam to send a message that the killing of his brother was not hidden from Allah.
Ancient Greek authors tell how a jackdaw, being a social creature, may be caught with a dish of oil that it falls into while looking at its own reflection. The Roman poet Ovid saw them as a harbinger of rain (Amores 2,6, 34). 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, who still seeks shiny things.
In Aesop's Fables, the jackdaw embodies stupidity in one tale, by starving while waiting for figs on a fig tree to ripen, and vanity in another – the jackdaw sought to become king of the birds with borrowed feathers, but was shamed when they fell off. Pliny notes 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. Another ancient Greek and Roman adage runs, "The swans will sing when the jackdaws are silent," meaning that educated or wise people will speak after the foolish become quiet. In reality, corvids are among the most intelligent birds in the world, and this traditional association with ignorance is quite inaccurate. However, there is one other Aesop Fable where the crow is depicted as very cunning. He comes up to a pitcher and knows that his beak is too short to reach the water and if he tips it over, all the water will fall out. The crow then proceeds to pick up pebbles and places them in the pitcher so the water may rise and he can reach it to relieve his thirst.
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