Corvus is a widely distributed genus of passerine birds ranging from medium-sized to large-sized in the family Corvidae. It includes species commonly known as crows, ravens, and rooks. The species commonly encountered in Europe are the carrion crow, hooded crow, common raven, and rook; those discovered later were named "crow" or "raven" chiefly on the basis of their size, crows generally being smaller. The genus name is Latin for "raven".
The 46 or so members of this genus occur on all continents except South America, Antarctica and several islands. The genus Corvus 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".
In 2002, research found that some crow species are 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.
Description
Medium-large species are ascribed to the genus, ranging from of some small Mexican species to of the large common raven and thick-billed raven, which together with the lyrebird represent the larger passerines.
These are birds with a robust and slender appearance, equipped with a small, rounded head with a strong, conical beak, elongated and pointed, with a slightly curved end towards the bottom; the legs are strong and the tail is short and wedge-shaped.
The coloration of the livery is dominated by shades of black, with some species having plumage with metallic iridescence and others that have white or gray areas on the neck or torso. Australian species have light eyes, while generally the irises of other species are dark.
Sexual dimorphism is limited.
Evolutionary history and systematics
The members of the genus Corvus are believed to have evolved in Central Asia and radiated out from there into North America, Africa, Europe, and Australia. The center of diversity of Corvus is within Melanesia, Wallacea, and the island of New Guinea and surrounding islands, with numerous species endemic to islands in the area; other areas with a large number of crow species include South and Southeast Asia, East Africa, and Australia. A high density of endemics is also present in Mexico and the Caribbean.
The diversification of Corvus corresponded with a quick geographic expansion. The radiation of the genus resulted in rapid expansion of morphological diversity and fast speciation rates, especially around the beginning of the genus' radiation around 10 million years ago.
The fossil record of crows is rather dense in Europe, but the relationships among most prehistoric species are not clear. Early Pleistocene fossils of crows indeterminate to the species level are known from the Nihewan Basin of China.
The genus was originally described by Carl Linnaeus in his 1758 10th edition of Systema Naturae. The name is derived from the Latin corvus meaning "raven". The type species is the common raven (Corvus corax); others named by Linnaeus in the same work include the carrion crow (C. corone), hooded crow (C. cornix), rook (C. frugilegus), and two species which have since been moved to other genera, the western jackdaw (now Coloeus monedula) and the Eurasian magpie (now Pica pica). At least 42 extant species are now considered to be members of Corvus, and at least 14 extinct species have been described.
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 the mid-1900s. Crows and ravens spread along with agriculture and urbanization into the western part of North America.Marzluff, J., Bowman, R. and Donnelly, R. (2001). Avian ecology and conservation in an urbanizing world. Boston: Kluwer Academic Publishers.
Species
Behavior
Communal roosting 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.Caccamise, D., Reed, L. and Stouffer, P. (1997). Roosting Behavior and Group Territoriality in American Crows. The Auk, 114(4), 628-637.
Play Countless incidents are recorded of corvids at play. Many behaviourists see play as an essential quality in intelligent animals.Bekoff, Mark & Byers, John (1998). Animal Play: Evolutionary, Comparative and Ecological Perspectives.
Calls
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).
Foraging
Along with other birds, ravens have been known to associate with other animals such as coyotes and wolves. These associations are linked to feeding and hunting. Ravens use their calls to notify these animals when an injured prey is near. This interaction is most noticeable in winter where ravens are associated with wolf packs nearly 100% of the time. As a result of this connection, studies have been conducted on the reaction of prey animals to the call of the raven. In areas where ravens associate with predators, prey animals are more likely to avoid predation by leaving after hearing the call. Crows are also capable of distinguishing between coyotes and wolves and have shown a preference for wolves. This may be due to the fact that wolves kill larger prey. Crows also frequently mob owls, especially great horned owls, which are significant predators of corvids. This behavior involves loud calling and repeated dive-bombing to drive the owl out of the area and reduce danger to nests and fledglings. When hunting, ravens can locate injured animals, like elk, and can call out to wolves to kill them. At times, ravens associate with wolves even when there is no carcass and can even be seen forming relationships with them. This includes playing with cubs by using sticks, picking at their tails, or flying around them.
Ravens have been mostly seen among travelling wolf packs rather than resting wolves, possibly due to the increased likelihood of food. They are also known to trust wolves in the pack they follow; when encountering a carcass killed by animals other than wolves, they are more apprehensive to eat from it. This symbiotic relationship between ravens and wolves is shown to be mutualistic; ravens help wolves find prey and when the wolves kill them the ravens can eat too. However, this relationship is not without its faults. Ravens may sometimes eat more of the prey than the wolf does. This problem has also been linked to wolf pack size, with some researchers suggesting that one of the reasons wolves hunt in larger packs is so that ravens (and other scavengers) get less of the food. Along with contention in wolves, ravens can also bother each other. By feeding off of the same carcass it is possible that some ravens will steal from their conspecifics. This behaviour is related to the ravens' ability to make quick decisions about eating the food then or storing it for later, and to their dominance and fighting ability.
Intelligence 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 midair 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 proximal 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.Schmid, Randolph E. (5 October 2007) , Associated Press via Discovery ChannelSee also the video "Crow bars", from the BBC's The Life of Birds Crows in Queensland have learned how to eat the toxic cane toad by flipping the cane toad on its back and 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 western jackdaw and the Eurasian 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 gibbons. Research has shown that corvids possess unusually high neuron density in their forebrains, which contributes to cognitive abilities comparable to those of some primates.Caffrey, Carolee. “Bird Brainpower.” Skeptic, vol. 29, no. 1, 2024, pp. 30–35.
Crows have demonstrated the ability to distinguish individual humans by recognizing facial features. A neuroimaging study demonstrated that crows recognize individual human faces and activate brain regions associated with perception, attention, and fear when viewing threatening individuals. Evidence also suggests they are one of the few nonhuman animals, along with insects like bees or ants, capable of displacement (communication about things that are not immediately present, spatially or temporally).
In the Gumyoji Park of Yokohama, Japan, crows have shown the ability to both activate public drinking fountains and adjust the water flow to appropriate levels for either bathing or drinking.