0 the american crow (corvus brachyrhynchos) is a large passerine bird species of the family corvidae .
1  it is a common bird found throughout much of north america .
2  american crows are the new world counterpart to the carrion crow and the hooded crow .
3  although the american crow and the hooded crow are very similar in size , structure and behavior , their calls are different .
4  the american crow nevertheless occupies the same role the hooded crow does in eurasia .
5 from beak to tail , an american crows measures 4050 cm (1620 in), almost half of which is tail .
6  mass varies from about 300 to 600 g (10 to 20 oz) .
7  males tend to be larger than females .
8  the most usual call is caaw-caaw-caaw .
9 the american crow is all black , with iridescent feathers .
10  it looks much like other all-black corvids .
11  they can be distinguished from the common raven (c .
12  corax) because american crows are smaller and from the fish crow (c .
13  ossifragus) because american crows do not hunch and fluff their throat feathers when they call .
14 american crows are common , widespread , and susceptible to the west nile virus , making them useful as a bioindicator to track the virus's spread .
15  direct transmission of the virus from american crows to humans is unheard of and unlikely .
16 the american crow was described by christian ludwig brehm in 1822. its scientific name means literally "short-billed crow", from ancient greek brachy- (-) "short-" and rhynchos () "billed" .
17 the northwestern crow (c .
18  caurinus) is very closely related to the american crow .
19  its ancestors became separated by ice age glaciation west of the rocky mountains .
20  it is endemic to pacific temperate rain forests where it all but replaces the american crow .
21  only in the seattle region do they co-occur to any extent .
22  in form the two species are much alike .
23  there is a marked difference in voice .
24 four subspecies are recognized .
25  they differ in bill proportion and form a rough ne-sw clinal in size across north america .
26  birds are smallest in the far west and on the south coast .
27 the american crow is a distinctive bird with iridescent black feathers all over .
28  its legs , feet and bill are also black .
29  they are 4050 cm (1620 in) in length , of which the tail makes up about 40% .
30  each wing is around 2734 cm (1114 in) long .
31  the bill length is on average 5 cm (2 in), varying strongly according to location .
32  males tend to be larger than females .
33 the most usual call is a loud , short , and rapid caaw-caaw-caaw .
34  usually , the birds thrust their heads up and down as they utter this call .
35  american crows can also produce a wide variety of sounds and sometimes mimic noises made by other animals , including other birds .
36 visual differentiation from the fish crow (c .
37  ossifragus) is extremely difficult and often inaccurate .
38  nonetheless , differences apart from size do exist .
39  fish crows tend to have more slender bills and feet .
40  there may also be a small sharp hook at the end of the upper bill .
41  fish crows also appear as if they have shorter legs when walking .
42  more dramatically , when calling , fish crows tend to hunch and fluff their throat feathers .
43 if seen flying at a distance from where size estimates are unreliable , the distinctly larger common ravens (c .
44  corax) can be distinguished by their almost lozenge-shaped tail , their larger-looking heads and of course their strongly solitary habits .
45  they also fluff their throat feathers when calling like fish crows , only more so .
46 most wild american crows live for about 78 years .
47  captive birds are known to have lived up to 30 years .
48 the range of the american crow 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 .
49  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 raven .
50  this crow is a permanent resident in most of the usa , but most canadian birds migrate some distances southward in winter .
51  outside of the nesting season these birds often gather in large communal roosts at night .
52 the american crow was recorded in bermuda from 1876 onwards .
53 the american crow is omnivorous .
54  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 .
55  american crows are active hunters and will prey on mice , frogs , and other small animals .
56  in winter and autumn , the diet of american crows is more dependent on nuts and acorns .
57  occasionally , they will visit bird feeders .
58  the american crow is one of only a few species of bird that has been observed modifying and using tools to obtain food .
59 like most crows , they will scavenge at landfills , scattering garbage in the process .
60  where available , corn , wheat and other crops are a favorite food .
61  these habits have historically caused the american crow to be considered a nuisance .
62  however , it is suspected that the harm to crops is offset by the service the american crow provides by eating insect pests .
63 american crows are monogamous cooperative breeding birds .
64  mated pairs form large families of up to 15 individuals from several breeding seasons that remain together for many years .
65  offspring from a previous nesting season will usually remain with the family to assist in rearing new nestlings .
66  american crows do not reach breeding age for at least two years .
67  most do not leave the nest to breed for four to five years .
68 the nesting season starts early , with some birds incubating eggs by early april .
69  american crows build bulky stick nests , nearly always in trees but sometimes also in large bushes and , very rarely , on the ground .
70  they will nest in a wide variety of trees , including large conifers , although oaks are most often used .
71  three to six eggs are laid and incubated for 18 days .
72  the young are usually fledged by about 35 days after hatching .
73  predation primarily occurs at the nest site and eggs and nestlings are frequently eaten by snakes , raccoons , ravens and domestic cats .
74  adults are less frequently predated but face potential attack from great horned owls , red-tailed hawks , peregrine falcons and eagles .
75  they may be attacked by predators such as coyotes or bobcats at carrion when incautious although this is even rarer .
76 american crows succumb easily to west nile virus infection .
77  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 .
78  it is estimated that the american crow population has dropped by up to 45% since 1999; the disease runs most rampant in the subtropical conditions which encourage reproduction of its mosquito vectors among which culex tarsalis is most significant .
79  mortality rates appear to be higher than those in other birds , causing local population losses of up to 72% in a single season .
80  because of this , american crows are a sentinel species indicating the presence of west nile virus in an area .
81  crows cannot transmit the virus to humans directly .
82 crows have been killed in large numbers by humans , both for recreation and as part of organized campaigns of extermination .
83 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 .
84  the number of individual american crows is estimated by birdlife international to be around 31,000,000. the large population , as well as its vast range , are the reasons why the american crow is considered to be of least concern , meaning that the species is not threatened .
