About Corvus corax Linnaeus, 1758
A mature common raven (Corvus corax Linnaeus, 1758) measures 54 to 71 cm (21 to 28 in) in length, with a wingspan of 116 to 153 cm (46 to 60 in). Recorded body weights range from 0.69 to 2.250 kg (1.52 to 4.96 lb), making the common raven one of the heaviest passerine bird species. Individuals from colder regions such as the Himalayas and Greenland are generally larger, with slightly larger bills, while those from warmer regions are smaller with proportionally smaller bills. As an example of this species' size variation, ravens from California have an average weight of 0.784 kg (1.73 lb), those from Alaska average 1.135 kg (2.50 lb), and those from Nova Scotia average 1.230 kg (2.71 lb). The common raven has a large, slightly curved bill with a culmen length of 5.7 to 8.5 cm (2.2 to 3.3 in), which is one of the largest bills among passerines—only the thick-billed raven and white-necked raven have larger bills. It has a fairly long, strongly graduated tail that measures 20 to 26.3 cm (7.9 to 10.4 in), mostly iridescent black plumage, and a dark brown iris. Its throat feathers are elongated and pointed, and the bases of its neck feathers are pale brownish-grey. The legs and feet are stout and strong, with a tarsus length of 6 to 7.2 cm (2.4 to 2.8 in). Juvenile plumage is similar to that of adults but duller, with a blue-grey iris and a pinkish gape when young. Apart from its larger overall size, the common raven can be distinguished from related crows by its larger, heavier black beak, shaggy feathers around the throat, longer bristles above the beak, and a longer, wedge-shaped tail. When flying, common ravens can be told apart from crows by their tail shape, larger wing area, and more stable soaring style that generally involves less wing flapping. Despite their larger bulk, ravens are just as agile in flight as their smaller crow relatives. In flight, the raven's feathers produce a creaking sound that has been compared to the rustle of silk. The common raven's voice is also quite distinct; its usual call is a deep croak with a much more sonorous quality than a crow's call. However, the calls of other raven species such as the fan-tailed raven and brown-necked raven can be confused with the common raven's calls where all three occur together in parts of southwest Asia and northern Africa. Of these two species, the fan-tailed raven has a more similar call to the common raven but has a very different shape, with broad wings and a very short tail. The brown-necked raven can be very hard to tell apart from the common raven by plumage alone, but it has calls that are more similar to a crow's. In North America, the Chihuahuan raven (C. cryptoleucus) is fairly similar to the relatively small common ravens of the American southwest, and is best distinguished by its still relatively smaller bill, throat feather patch, and body, along with a relatively longer tail. In Europe, the all-black carrion crow (C. corone) and rook (C. frugilegus) may be mistaken for a raven due to their fairly large bills, but they are distinctly smaller and have the wing and tail shapes typical of crows. In the Faroe Islands, an extinct pied colour morph of this species once existed, known as the pied raven; ordinary black common ravens remain widespread across the archipelago. White ravens are occasionally found in the wild. Some white ravens recorded in British Columbia lacked the pink eyes of an albino, and were instead leucistic—a condition where an animal lacks several different types of pigment, not just melanin. Common ravens produce a wide range of vocalizations that are of interest to ornithologists. Gwinner conducted important studies of these vocalizations in the early 1960s, recording and photographing his findings in great detail. Between 15 and 30 categories of calls have been recorded for this species, most of which are used for social interaction. Recorded call types include alarm calls, chase calls, and flight calls. The species has a distinctive, deep, resonant prruk-prruk-prruk call that, to experienced listeners, is unlike that of any other corvid. Its very wide and complex vocabulary includes a high, knocking toc-toc-toc, a dry, grating kraa, a low guttural rattle, and some calls of an almost musical nature. Like other corvids, the common raven can mimic sounds from its environment, including human speech. Non-vocal sounds produced by the common raven include wing whistles and bill snapping. Clapping or clicking sounds have been observed more often in females than in males. If a member of a mated pair is lost, its surviving mate will reproduce the calls of its lost partner to encourage the lost bird to return. The common raven can thrive in a wide variety of climates, and has the largest range of any member of its genus, as well as one of the largest ranges of any passerine. Its range covers the entire Holarctic, from Arctic and temperate habitats in North America and Eurasia to the deserts of North Africa, and extending to islands in the Pacific Ocean. In the British Isles, common ravens are more common in Scotland, Wales, western England, and Ireland, having been eradicated from other areas by gamekeeping activities. In Tibet, they have been recorded at altitudes up to 5,000 m (16,400 ft), and as high as 6,350 m (20,600 ft) on Mount Everest. A population sometimes called the 'Punjab raven', part of the subspecies Corvus corax laurencei (sometimes incorrectly named C. c. subcorax), occurs in the Sindh district of Pakistan and adjoining regions of northwestern India. Common ravens are generally year-round residents within their range. In his 1950 work Grønlands Fugle [Birds of Greenland], noted ornithologist Finn Salomonsen stated that common ravens did not overwinter in the Arctic. However, common ravens are found year-round in Arctic Canada and Alaska. Young birds may disperse locally. In the United Kingdom, the species' range is currently increasing following improved legal protection, but illegal persecution by gamekeepers remains a problem in many areas. The common raven favours mountainous or coastal terrain, but can also be found in parks with tall trees suitable for use as shelter. Its population is densest in the north and west of the United Kingdom, though the species is expanding its population southwards. Most common ravens prefer wooded areas with large expanses of open land nearby, or coastal regions for their nesting sites and feeding grounds. In some areas with dense human population, such as California in the United States, they take advantage of a plentiful food supply and have seen a surge in their numbers. On coasts, individual common ravens are often evenly distributed and prefer to build their nest sites along sea cliffs. Common ravens are often found in coastal regions because these areas provide easy access to water and a variety of food sources, and have stable weather patterns without extreme cold or hot temperatures. In general, common ravens live in a wide array of environments but prefer heavily contoured landscapes. When the environment changes to a large degree, these birds respond with a stress response: the hormone corticosterone is activated by the hypothalamic–pituitary–adrenal axis, and corticosterone is activated when the bird is exposed to stress such as migrating long distances.
