User:Ms0615/American crow

The American crow (Corvus brachyrhynchos) is a large passerine bird species of the family Corvidae. It is a common bird found throughout much of North America. American crows are the New World counterpart to the carrion crow and the hooded crow. Although the American crow and the hooded crow are very similar in size, structure and behavior, their calls are different. The American crow, nevertheless, occupies the same role that the hooded crow does in Eurasia.

From beak to tail, an American crow measures 40 –, almost half of which is tail. Mass varies from about 300 to 600 g. Males tend to be larger than females. The most usual call is CaaW!-CaaW!-CaaW!.

The American crow is all black, with iridescent feathers. It looks much like other all-black corvids. They can be distinguished from the common raven (C. corax) because American crows are smaller, from the fish crow (C. ossifragus) because American crows do not hunch and fluff their throat feathers when they call and from the carrion crow (C. corone) by the enunciation of their calls.

American crows are common, widespread, and susceptible to the West Nile virus, making them useful as a bioindicator to track the virus's spread. Direct transmission of the virus from crows to humans is impossible.

Taxonomy
The American crow was described by Christian Ludwig Brehm in 1822. Its scientific name means literally 'short-billed crow', from Ancient Greek βραχυ- brachy- ('short-') and ρυνχος rhynchos ('billed').

Subspecies
Five subspecies are recognized. They differ in bill proportion and form a rough NE–SW clinal in size across North America. Birds are smallest in the far west and on the southern coast.


 * Corvus brachyrhynchos brachyrhynchos – eastern crow: northeastern United States, eastern Canada and surroundings. Largest subspecies.
 * Corvus brachyrhynchos caurinus – northwestern crow: Its ancestors became separated by Ice Age glaciation west of the Rocky Mountains. It is endemic to Pacific temperate rain forests where it all but replaces the American crow. Only in the Seattle region do they co-occur to any extent. There is a marked difference in voice.
 * Corvus brachyrhynchos hesperis – western crow: western North America except the Arctic north, the Pacific Northwest and the extreme south. Smaller overall with a proportionally more slender bill and low-pitched voice.
 * Corvus brachyrhynchos pascuus – Florida crow: Florida. Mid-sized, short-winged, but decidedly long bill and legs.
 * Corvus brachyrhynchos paulus – southern crow: southern United States. Smaller overall, bill also small.

Description
The American crow is a large, distinctive bird with iridescent black feathers all over. Its legs, feet and bill are also black. They measure 40 – in length, of which the tail makes up about 40%. The wing chord is 24.5 to 33 cm, with the wingspan ranging from 85 to 100 cm. The bill length can be from 3 to 5.5 cm, varying strongly according to location. American crows have light gray or blue eyes as juveniles that will become darker as they reach adulthood. The tarsus is 5.5 to 6.5 cm and the tail is 13.5 to 19 cm. The body mass can vary from 316 to 620 g. Males tend to be larger than females. The most usual call is a loud, short, and rapid caaw-caaw-caaw. Usually, the birds thrust their heads up and down as they utter this call. American crows can also produce a wide variety of sounds and sometimes mimic noises made by other animals, including other birds like barred owls.

Visual differentiation from the fish crow (C. ossifragus) is extremely difficult and often inaccurate. Nonetheless, differences apart from size do exist. Fish crows tend to have more slender bills and feet. There may also be a small sharp hook at the end of the upper bill. Fish crows also appear as if they have shorter legs when walking. More dramatically, when calling, fish crows tend to hunch and fluff their throat feathers.

If seen flying at a distance from where size estimates are unreliable, the distinctly larger common ravens (C. corax) can be distinguished by their almost lozenge-shaped tail and their larger-looking heads. They also fluff their throat feathers when calling — like fish crows, only more so. Ravens also soar for extended periods, unlike crows, which rarely fly more than a few seconds without flapping their wings.

Crows have been noted to be intelligent. They have the same brain-weight-to-body ratio as humans. This has led to some studies that have identified that crows are self-aware and that young crows take time to learn from tolerant parents. While a human has a neocortex, the crow has a different area in their brain that is equally complex.

The average lifespan of the American crow in the wild is 7–8 years. Captive birds are known to have lived up to 30 years.

Distribution and habitat
The range of the American crow now extends from the Pacific Ocean to the Atlantic Ocean in Canada, on the French islands of Saint-Pierre and Miquelon, south through the United States, and into northern Mexico. The increase in trees throughout the Great Plains during the past century due to fire suppression and tree planting facilitated range expansions of the American crow as well as range expansions of many other species of birds. Virtually all types of country from wilderness, farmland, parks, open woodland to towns and major cities are inhabited; it is absent only from Pacific temperate rain forests and tundra habitat, where it is replaced by the common raven. This crow is a permanent resident in most of the US, but most Canadian birds migrate some distances southward in winter. Outside of the nesting season these birds often gather in large (thousands or even millions ) communal roosts at night.

The American crow was recorded in Bermuda from 1876 onwards.

Diet
The American crow is omnivorous. It will feed on invertebrates of all types, carrion, scraps of human food, seeds, eggs and nestlings, stranded fish on the shore and various grains. American crows are active hunters and will prey on mice, frogs, and other small animals. In the winter and autumn, the diet of American crows is more dependent on nuts and acorns. Occasionally, they will visit bird feeders. The American crow is one of only a few species of bird that has been observed modifying and using tools to obtain food.

Like most crows, they will scavenge at landfills, scattering garbage in the process. Where available, corn, wheat and other crops are a favorite food. These habits have historically caused the American crow to be considered a nuisance. However, it is suspected that the harm to crops is offset by the service the American crow provides by eating insect pests.

Reproduction
American crows are socially monogamous cooperative breeding birds. Mated pairs form large families of up to 15 individuals from several breeding seasons that remain together for many years. Offspring from a previous nesting season will usually remain with the family to assist in rearing new nestlings. American crows do not reach breeding age for at least two years. Most do not leave the nest to breed for four to five years.

The nesting season starts early, with some birds incubating eggs by early April. American crows build bulky stick nests, nearly always in trees but sometimes also in large bushes and, very rarely, on the ground. They will nest in a wide variety of trees, including large conifers, although oaks are most often used. Three to six eggs are laid and incubated for 18 days. The young are usually fledged by about 36 days after hatching. Predation primarily occurs at the nest site and eggs and nestlings are frequently eaten by snakes, raccoons, ravens and domestic cats. Adults are less frequently predated, but face potential attack from great horned owls, red-tailed hawks, peregrine falcons and eagles. They may be attacked by predators such as coyotes or bobcats at carrion when incautious, although this is even rarer. During breeding and nesting season, crows are likely to engage in mobbing behavior. This behavior takes place when pairs or families of crows feel as though their nests or offspring are threatened by an approaching predator. Adult crows will aggressively harass a predator until it leaves their established breeding territory.

Intelligence
Crows are one of the most intellectually advanced species of birds. As previously mentioned, they have a large brain to body ratio—much of which is accounted for in the forebrain. This area of a crow’s brain is comparable to that of a primate’s and is responsible for complex cognitive functions. Some of these include problem-solving, regulating behavior, and the processing of sensory information.

A man by the name of John M. Marzluff has dedicated nearly 40 years to studying Corvid behavior. In his research, he tested the ability of crows to recognize human faces. In Marzluff's study, crows were exposed to either positive or negative experiences associated with a specific mask worn by researchers. The birds would then perceive that individual as either threatening or nonthreatening. PET scans were used during the experiment to detect activity in certain areas of the brain. This study revealed that crows process emotion and fear-based learning very similarly to humans and other intelligent species of vertebrates.

West Nile virus
American crows succumb easily to West Nile virus infection. This was originally a mosquito-borne African virus causing encephalitis in humans and livestock since about 1000 AD, and was accidentally introduced to North America in 1999, apparently by an infected air traveller who got bitten by a mosquito after arrival. It is estimated that the American crow population has dropped by up to 45% since 1999. Despite this decline, the crow is considered a species of least concern. The disease runs most rampant in the subtropical conditions which encourage reproduction of its mosquito vectors among which Culex tarsalis is most significant. Mortality rates appear to be higher than those in other birds, causing local population losses of up to 72% in a single season. Because of this, American crows are a sentinel species indicating the presence of West Nile virus in an area. Crows cannot transmit the virus to humans directly.

Status and conservation
Crows have been killed in large numbers by humans, both for recreation and as part of organized campaigns of extermination.

American crows are protected internationally by the Migratory Bird Treaty Act of 1918. Despite attempts by humans in some areas to drive away or eliminate these birds, they remain widespread and very common. In 2012, BirdLife International estimated the American crow population to be around 31 million. The large population and vast range result in the least concern status for the American crow, meaning that the species is not threatened.