American crow
Template:Short description Template:Speciesbox The American crow (Corvus brachyrhynchos) is a large passerine bird species of the family Corvidae. They are a common bird found throughout much of North America, only absent from the tundra biome. American crows are the New World counterpart to the carrion crow and the hooded crow of Eurasia; all occupying the similar ecological niches.
American crows have all black plumage, with iridescent feathers. It looks much like other all-black corvids. They are highly intelligent, similar to other Corvidae, and are adaptable to human environments. The most common call of an American crow is the Template:Not a typo They can be distinguished from the common raven by their smaller size and a slightly less pronounced beak; from the carrion crow by their size, as the carrion crow is larger and of a stockier build. American crows can be difficult to distinguish from the fish crow, however fish crows tend to fluff their throat feathers when calling. They are omnivores, feeding on insects, seeds, nuts, and preying on small animals. They also rarely feed on carrion, leading to associations with death, thieves, and bad luck in some cultures, however, in Native American, along with some Gaelic cultures, crows are considered good luck, due to their intelligence.
American crows are common, widespread, and susceptible to the West Nile virus, making them a useful bioindicator for tracking the virus's spread. Direct transmission of the virus from crows to humans is impossible. According to BirdLife International, there are 31 million American crows as of 2012.
Taxonomy and systematics
The American crow was described by German ornithologist Christian Ludwig Brehm in 1822.<ref name="Verbeek-2021" /> Its scientific name means literally 'short-billed crow', from Ancient Greek Template:Lang Template:Lang ('short-') and Template:Lang Template:Lang ('billed').<ref name=Liddell/>
A 2012 genetic analysis of the genus Corvus calculated that the American crow diverged from a lineage that gave rise to the collared, carrion and hooded crows around 5 million years ago.<ref name="jonsson 2012">Template:Cite journal</ref>
"American crow" has been designated the official name by the International Ornithologists' Union (IOU).<ref name="ioclist">Template:Cite web</ref>
Subspecies
The number of subspecies varies by authority, ranging between three and five. The unclear taxonomy of the northwestern crow, previously its own species, has complicated subspecies determinations. Subspecies 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.<ref name="m&b1994" /><ref name="Verbeek-2021">Template:Cite journal</ref>
- C. b. brachyrhynchos (Brehm, 1822) – eastern crow: northeastern United States, eastern Canada and surroundings. The nominate subspecies, and largest.<ref>Template:Cite book</ref>
- C. b. hesperis (Ridgway, 1887) – western crow: western North America except the Arctic north, the Pacific Northwest and the extreme south. Smaller overall with a proportionally more slender bill<ref name="g&g87">Goodwin & Gillmor (1976): p. 87</ref> and low-pitched voice.
- C. b. caurinus (Baird, 1858) – northwestern crow: of the Pacific temperate rain forests was formerly considered a distinct species as C. caurinus, averaging smaller in size than other American crows with a distinctly hoarser call.<ref name="Dick" /> It forms a hybrid swarm with American crow (sensu stricto) in coastal Washington and British Columbia.<ref name="Pyle, Peter 1997">Template:Cite journal</ref> In 2020, the American Ornithological Society reclassified the Northwestern Crow as conspecific with the American Crow.<ref>Template:Cite journal</ref> It is now considered a geographic variation within C. b. hesperis.<ref>Template:Cite web</ref>
- C. b. pascuus (Coues, 1899) – Florida crow: Florida. Mid-sized, short-winged, but decidedly long bill and legs.<ref name="g&g87" />
- C. b. paulus (Howell, 1913) – southern crow: southern United States. Smaller overall, bill also small.<ref>Goodwin & Gillmor (1976) p. 88.</ref>
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 Template:Convert in length, of which the tail makes up about 40%. The wing chord is Template:Convert, with the wingspan ranging from Template:Convert. The bill length can be from Template:Convert, varying strongly according to location. The tarsus is Template:Convert and the tail is Template:Convert.<ref name="m&b1994" /> The body mass can vary from Template:Convert. Males tend to be larger than females.<ref>Kilham, Lawrence. The American Crow and the Common Raven. p. 52</ref><ref>American Crow, Life History, All About Birds – Cornell Lab of Ornithology. Allaboutbirds.org. Retrieved on 2013-03-06.</ref> The most usual call is a loud, short, and rapid Template:Not a typo. 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, such as barred owls.<ref name="Kilham1990">Template:Cite book</ref>
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 fish crow's 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.<ref>Template:Cite web</ref><ref>Template:Cite web</ref><ref>Template:Cite web</ref>
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.<ref>Template:Cite web</ref><ref>Template:Cite web</ref> They also fluff their throat feathers when calling, similarly to fish crows.<ref>Template:Cite web</ref>
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Ravens also soar for extended periods, unlike crows, which rarely fly more than a few seconds without flapping their wings.<ref>Template:Cite web</ref><ref>Template:Cite web</ref>
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 uses a different area in its brain, the pallium,<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> to perform similar tasks.<ref>Template:Cite journal</ref><ref>Template:Cite web</ref> They are able to remember and identify people based on their appearance.<ref>Template:Cite web</ref><ref>Template:Cite web</ref>
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.<ref name="cdep2001" /> The West Nile virus is a major cause of death in crows.<ref>Template:Cite journal</ref>
Distribution and habitat
The American crow has a large range, extending 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.<ref name="iucn" /> They have also been recorded in Bermuda from 1876 onwards.<ref>Template:Cite book</ref> 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<ref>Houston S. 1977. Changing patterns of Corvidae on the prairies. Blue Jay 35:149–155.</ref> as well as range expansions of many other species of birds.<ref>Livezey KB. 2009a. Range expansion of Barred Owls, part I: chronology and distribution. American Midland Naturalist 161:49–56.</ref><ref>Livezey KB. 2009b. Range expansion of Barred Owls, part 2: facilitating ecological changes. American Midland Naturalist 161:323–349.</ref><ref>Livezey KB. 2010. Killing barred owls to help spotted owls II: implications for many other range-expanding species. Northwestern Naturalist 91:251–270.</ref> Virtually all types of country from wilderness, farmland, parks, open woodland to towns and major cities are inhabited; it is absent only from tundra habitat, where it is replaced by the common raven. The American crow is a permanent resident in most of the US, but most Canadian birds migrate some distances southward in winter.<ref>Template:Cite web</ref> Outside of the nesting season these birds often gather in large communal roosts of thousands or even millions of crows at night.<ref>Template:Cite web</ref><ref name="Caccamise Reed Stouffer 1997 pp. 628–637">Template:Cite journal</ref>
Behavior and ecology
Studying the behavior of American crows is laborious due to the difficulty in catching them, resulting in much of their behavior, including daily routine, migration, molting, survivorship, age of first breeding, nestling development, and the nature of nesting helpers being poorly studied.<ref name="Verbeek-2021" /><ref>Template:Cite journal</ref>
Diet
The American crow is omnivorous.<ref>Template:Cite web</ref> It will feed on invertebrates of all types, carrion, scraps of human food, fruits,<ref name="auto">Template:Cite web</ref> nuts such as walnuts and almonds, seeds, eggs and nestlings, stranded fish on the shore and various grains. American crows are active hunters and will prey on mice, young rabbits,<ref name="auto" /> 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.<ref name = parr2005/> The American crow is one of only a few species of bird that has been observed modifying and using tools to obtain food.<ref name=Caffrey2000/>
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.<ref name = parr2005 />
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.<ref name="segelken">Roger Segelken: Tree-climbing researcher knows exactly how far the crow flies Template:Webarchive Cornell Chronicle. Retrieved 2006-October-25,</ref> 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.<ref name=CLO/> Most do not leave the nest to breed for four to five years.<ref name = segelken />
The nesting season starts early, with some birds incubating eggs by early April.<ref name=Henninger/> 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.<ref>Template:Cite web</ref><ref name=":0">Template:Cite web</ref> 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.<ref name=":0" /> They may be attacked by predators such as coyotes or bobcats at carrion when incautious, although this is even rarer.<ref name="parr2005" /><ref>Template:Cite web</ref> American crows have been shown to be more wary of disturbances in urban environments.<ref>Template:Cite journal</ref> The American crow experiences inbreeding depression, yet research indicates they maintain a preference for such behavior.<ref name="Townsend2019">Template:Cite journal</ref>
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 air traveller who was infected by a mosquito bite after arrival. It is estimated that the American crow population has dropped by up to 45% since 1999.<ref>Template:Cite journal</ref> Despite this decline, the crow is considered a species of least concern.<ref>Template:Cite news</ref> 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.<ref name=CLO/><ref name=Caffrey2005/> 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.<ref name = cdep2001 />
Intelligence
American crows, like other corvids, are highly cunning and inquisitive. They are able to steal food from other species, often in creative ways. One example shows a group of crows stealing a fish from a Northern river otter: one bird pecked the otter's tail to distract it while other birds swooped in and stole the fish.<ref name="Verbeek-2021" /> American crows have been observed using and modifying tools,<ref>Template:Cite journal</ref> however, unlike the Hawaiian and New Caledonian crow, this behavior is not inherent.<ref>Template:Cite web</ref> Female crows have been observed to be better at using tools, according to a 2023 study.<ref>Template:Cite journal</ref> American crows have also been noted for their ability to play, such as snowboarding using tree bark.<ref>Template:Cite web</ref><ref>Template:Cite web</ref>
Relationship with humans
Crows are used as a motif in some human cultures, often associated with death, thieves, graveyards, bad luck, and other negative connotations.<ref name="Sullivan">Template:Cite web</ref> American crows are sometimes considered a pest, often due to their loud calls.<ref>Template:Cite web</ref><ref>Template:Cite web</ref> However, in other cultures, like Native American folklore, crows are considered good luck, noted for their intelligence.<ref>Template:Cite web</ref> Similarly, they are also seen by some neo-pagan and indigenous cultures as signs of good luck, or even signs of certain gods, such as Apollo, Odin, and others.<ref>Template:Cite web</ref> Crows are also featured in Aesop's Fables, in stories such as the The Crow and the Pitcher, and The Fox and the Crow.<ref>Template:Cite web</ref><ref>Template:Cite web</ref>
Status and management
The intelligence and adaptability of the American crow has insulated it from threats, and it is instead considered an agricultural pest.<ref name="Verbeek-2021" /> 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 with extinction.<ref name="iucn" />
Crows have been killed in large numbers by humans, both for recreation and as part of organized campaigns of extermination.<ref name="Campbell" /> In Canada, American crows have no protections, aside from Quebec, which bans their hunting during the nesting season.<ref name="Verbeek-2021" /> Laws on their hunting vary throughout the United States. New Jersey allows for a limited hunting season, unless they are agricultural pests, in which case they may be killed. Oklahoma allows hunting even during the nesting season. In the first half of the 20th century, state-sponsored campaigns dynamited roosting areas, taking large numbers of crows. A campaign in Oklahoma from 1934 to 1945 dynamited 3.8 million birds. The effect on populations was negligible and damage to agricultural crops did not decrease, and thus the campaign was halted due to its ineffectiveness. In a study taking data from 1917 to 1999, intentional killings were the overwhelming cause of death for crows, accounting for 68% of all recovered bird bands.<ref name="Verbeek-2021" />
Non-deadly methods of managing crows are varied but usually limited in their effectiveness. High value crops may be netted, but this is cost prohibitive for most other crops. Frightening may be used to disperse crows, including loud noises from guns, fake hawks flown from balloons, fake owls that move with the wind, strips of reflective tape on fences, or recordings of crow distress calls.<ref>Template:Cite web</ref> Crows quickly learn to avoid the less-toxic baits, as the baits make crows sick. The actual effect of crows on agriculture has been poorly studied.<ref name="Verbeek-2021" /> There is some suggestion that they may be a benefit to farmers, by eating insect pests and chasing off livestock predators like hawks.<ref name="Sullivan" />
References
Bibliography
- Goodwin, Derek & Gillmor, Robert (1976): Crows of the World (1st ed.). University of Washington Press, Seattle.
