Crows /kroʊ/ form the genus Corvus 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 for 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.
The crow genus makes up a third of the species in the Corvidae family. Crows 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, more poetically, 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 a brain-to-body size ratio approaching that of some apes. 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 make a wide variety of calls or vocalizations. Crows have also been observed to respond to calls of other species; this behavior is, it is presumed, 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 numerical vocalizations have been observed to change in response to events in the surroundings (i.e. arrival or departure of 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 its own tools in the day-to-day search for food. These tools include “knives” cut from stiff leaves and stiff stalks of grass. Another skill involves dropping tough nuts into a trafficked street and waiting for a car to crush them open. On October 5, 2007, researchers from the University of Oxford, England, presented data acquired by mounting tiny video cameras on the tails of New Caledonian Crows. It turned out that they use a larger variety of tools than previously known, plucking, smoothing, and bending 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.
Crows have demonstrated the ability to distinguish individual humans by recognizing facial features.