Corvidae

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{{Taxobox | color = pink | name = Corvidae | image = Kappenblaurabecele4.jpg | image_width = 240px | image_caption = Cyanocorax chrysops | regnum = Animalia | phylum = Chordata | classis = Aves | ordo = Passeriformes | familia = Corvidae | familia_authority = Vigors, 1825 | subdivision_ranks = Genera | subdivision =

}}

Corvidae is a family of oscine passerine birds that contains the crows, ravens, rooks, jackdaws, magpies, treepies and nutcrackers (Clayton and Emery 2005, [1]). Collectively its members are called corvids and there are over 120 species. These birds are well known for their intelligence, curiosity, playfulness and raucous calls.

Contents

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Genera and Species

Hume's Ground Jay (Pseudopodoces humilis) is a member of the Tit or Titmouse family (Paridae).


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Historic and current taxonomic status

The earliest corvid fossils date to about 17 million years ago, during the mid-Miocene period [2]. The genus Corvus, including the crows and ravens, makes up over a third of the entire family. The name Corvus was given to these birds due to their raucous “croaking” calls [3]. Corvids first arose in Australasia and from there, spread throughout the world. Early branches from these ancestors evolved into today's families such as currawongs, birds-of-paradise, whipbirds, quail-thrushes, whistlers, monarchs, shrikes, vireos and vangas [4].

Over the years there has been much discrepancy on the entirety of the corvid family. Despite Sibley and Ahlquist’s desire to include the early branches mentioned above, other authorities disagree, making the Family Corvidae much smaller [5].

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Typical size and appearance

Corvids have feathered rounded nostrils, strong tails and wings and similar sexes. Many corvids of temperate zones are mainly black, however, some are pied black and white, some have a blue-purple iridescence and many tropical species are highly colored. Corvids have strong, stout bills, large wingspans and are between 23-71cm long [6]. Members of the genus Corvus are the largest members of the song-bird order reaching 20-27 inches. Species can be identified based on size, shape and geography; however, they are also identified by their raucous calls. [7].

Easily confused are the crows and ravens. The ravens, on average, are larger than crows, with a large heavy beak and pointed “spiky” feathers. The best way to tell a crow from a raven, is by call. The raven’s caw is much deeper and throatier than the crow’s [8].

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Social interaction

Corvids harbor strong organization and community groups. Jackdaws, for example, have a strong social hierarchy, and are facultatively colonial during breeding (Verhulst and Salomons 2004). Providing mutual aid has also been recorded within many of the corvid species [9].

Young corvids have been known to play and take part in elaborate social games. The games resemble “king of the mountain” and “follow the leader” along with games that manipulate, pass and balance sticks. Corvids also take part in other fun activities, such as sliding down smooth surfaces, and these games are understood to play a large role in the adaptive and survival value of the birds (Gill 2003). Also, it is not unusual to see crows suspended from power lines upside down or performing aerobatic movements [10].

Corvids are highly aggressive birds. Blue Jays, for example, are well known to attack anything that threatens their nest. Crows have been known to attack dogs, cats, ravens, and birds of prey. Most of the time these assaults take place for a distraction long enough to allow the crow to steal food [11].

Human social interaction with the corvids is mostly detrimental, for the humans. Corvids raid agricultural crops and are predators of other birds. Therefore most farmers and birders scorn their existence (Clayton and Emery 2005).

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Food and foraging habits

The natural corvid diet is omnivorous, consisting of invertebrates, nestlings, small mammals, berries, fruits, seeds, and carrion. However, most corvids, especially the crows, have adapted well to human conditions and have come to rely on anthropogenic foods. In a study of crows, ravens and Steller’s Jays around campgrounds and human settlements, crows appeared to have the most diverse diet of all, taking part in anthropogenic foods such as bread, spaghetti, fried potatoes, dog food, sandwiches, and livestock feed. The increase in available anthropogenic food sources is contributing to population increase in some corvid species. (Marzluff and Neatherlin 2006).

During the wintering months, corvids typically form foraging flocks [12]. Crows in particular are renown for destroying crops, especially corn. However, crows also eat many agricultural pests including cutworms, wireworms, grasshoppers and harmful weeds [13]. The legendary desire of corvids for dead flesh is true, however, since corvids have no specialized beak for tearing into flesh, they must wait until animals decomposes before consumption [14].

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Migration

Migration is deterred by the availability of anthropogenic resources available year round. Therefore, most corvids are sedentary and do not migrate significantly. Corvids can survive many different types of climates, with their black attire providing additional heat in winter months [15]. However, during a shortage of food, erupted migration can occur [16]. When species are migratory, they will form large flocks in the fall (around August) and travel south [17].

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Types of nests/reproduction

Corvids worldwide are well known communal roosters. Some groups of roosting corvids have been as large as 2,000 birds (Everding and Jones 2006). The partner bond in corvids is extremely strong and even lifelong in some species. This monogamous lifestyle, however, can still contain extra pair copulations. Males and females build large nests together in trees or on ledges. The male will also feed the female during incubation [18]. The nests are constructed of a mass of bulky twigs lined with grass and bark. Corvids can lay between 3-10 eggs, typically ranging between 4-7. The eggs are usually greenish in color with brown blotches. Once hatched, the young remain in the nests for up to 6-10 weeks depending on the species. As expected, corvids provide biparental care.

Sexual selection is also quite complex in the Corvidae family. Young corvid members undergo a series of tests, including aerobatic feats, before being accepted as a mate by the opposite sex [19].

Unlike most other species, corvid fitness and reproduction, especially with the crows, has increased due to human development. The survival and reproductive success of crows and ravens, according to Marzluff and Neatherlin’s 2006 study, was positively associated with their intimacy of human populations. The corvid member most strongly associated with humans is the American crow. Human development provides additional resources by clearing land, creating shrublands rich in berries and insects. When the cleared land naturally replenishes, the young dense trees are used by jays and crows for nesting sites. Ravens typically use larger trees in denser forests (Marzluff and Neatherlin 2006).

One reason for the success of crows, compared to ravens, is their ability to overlap breeding territory. During breeding season, crows were shown to overlap breeding territory six times the overlap of ravens. This invasion of breeding ranges allowed a related increase in local density (Marzluff and Neatherlin 2006). In the US the American Crow population has definitely grown over the years. It is possible, that the American Crow, due to humans increasing suitable habitat, will drive out the Northwestern and Fish Crows (Marzluff and Angell 2005).

Jackdaws can breed in buildings or in rabbit warrens (Verhulst and Salomons 2004). White-throated Magpie-jays are cooperatively breeding corvids where the helpers are mostly female. Cooperative breeding takes place when additional adults help raise the nestlings. These adults are often called “helpers” and in most cooperatively breeding birds the males take on the “helper” role while females join other groups (Berg 2005).

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Nest predation

Since crows do not seem to mind human development, it was suggested that the crow population increase would cause increased rates of nest predation. However, the Steller’s Jays, which were successful independent of human development, were the more frequent nest predator. Therefore, the human relationship with crows and ravens did not increase nest predation since jays accounted for the most nest predation by corvids (Marzluff and Neatherlin 2006).

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Prejudice

Due to their typically black feathers, fondness of carrion, and history of trickery, humans have long since associated members of Corvidae with death and extreme injustice (Marzluff and Angell 2005). Throughout history, corvids have been negatively perceived as dark messengers, bearing ill will and other demonic associations. This dark connection is demonstration by the terms used to describe groups of crows (a murder), ravens (unkindness, constable or conspiracy), and jays (scold). [20].

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Myths

Despite the well-known demonic association, folklore also represents corvids as wise animals. Native Americans believed that a raven created the earth, the Norse god Odin constantly sought the advice of ravens, and even Aesop featured corvids as smart heros in many fables (Clayton and Emery 2005). According to native cultures, despite being a trickster spirit, ravens were popular on totems, were credited with creating man and were responsible for placing the Sun in the sky. In western literature, popularized by E.A. Poe, the Common Raven was a symbol of darkness, depression and death. However, in medieval times the raven stood for virility. Legends report that a raven’s favorite food is dead animals, and that they sometimes hunt with wolves [21]. For more myths and legends see crows and raven pages.

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Comparing within the family

Crows are long lived due to their great ability to coexist with, and adapt to, humans. Ravens, which are more secretive forest dwellers, are not fully tapping the anthropogenic resources along with being targets of human persecution (Marzluff and Neatherlin 2006). Therefore, over the years crow populations have increased while raven populations have decreased. One theory is based on crows taking part in social learning as part of their animal culture. The benefits of passing down information throughout generations has allowed crows to prosper (Marzluff and Angell 2005).


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Feathered apes

Corvids contain the largest brain, relative to their body size, of any bird. Based on a brain-to-body ratio, the corvid brain equals the size of a chimpanzee, is almost the same as a dolphin, and is only slightly lower than a human [22]. Their intelligence is evident due to the long developmental period of the young. By remaining with the parents, the young have more opportunities to learn necessary skills. Since most corvids are cooperative breeders, their young can learn from different members of the group (Clayton and Emery 2005). Some naturalists argue that the Corvidae family contains intelligence superior to all bird species [23]. When compared to other carnivorous mammals (specifically dogs and cats) in one laboratory test, corvid birds outshined their components, demonstrating operational abilities almost as excellent as monkeys (Krushinskii et al 1979). Dr. Louis Lefebvre’s avian IQ test declared Corvidae the most intelligent bird based on the scale [24].

The corvid ingenuity is represented through their feeding skills, memorization abilities, use of tools, and group behavior. Living in large social groups has long been connected with high cognitive ability. To live in a large group, a member must be able to recognize individuals and track the social position and foraging of other members over time. Members must also be able to distinguish between sex, age, reproductive status, dominance and be able to update the information constantly. Therefore, social complexity directly corresponds to high cognition (Bond et al 2003).

There are also specific examples of corvid cleverness. One crow was documented to crack nuts by placing them on a crosswalk, letting the passerby cars crack the shell, waiting for the light to turn red, and then safely retrieving the contents. A group of crows in England took turns lifting garbage bin lids while their companions collected food. Also, with their trickster reputations, its no wonder corvids are also occasionally thieves. Members of the corvid family have been known to watch other birds, remember where they hide their food, then return once the owner leaves. Corvids also move their food around between hiding places to avoid thievery, but only if they have previously been thieves themselves. The ability to hide food requires highly accurate spatial memories. Corvids have been recorded to recall their food’s hiding place up to nine months later. It is suggested that vertical landmarks (like trees) are used to remember locations. There has also been evidence that western scrub-jays, who store perishable foods, not only remember where they stored their food, but for how long. This is compared to human episodic memory, which was previously thought unique to humans (Clayton and Emery 2005).

Looking at the act of thievery in the corvid family, it has been suggested that birds will take their experience as a thief and use it to predict other bird actions of thievery. This explains why, if a corvid has committed thievery, they will take extra precautions (such as moving hiding places) to avoid being a future victim. Being able to predict others behavior based on your own experiences is another trait previously thought unique to humans. Laboratory experiments have confirmed that specifically crows, can sometimes use a past experience to approach a new obstacle (Clayton and Emery 2005).

Caledonian Crows have been observed to make tools of twigs trimmed into hooks. They then use to hooks to pull insect larvae from tree holes. These crows also use leaves to brush the ground to forage for prey beneath the soil. Caledonian crows are not the only corvids to use tools, and diversity in tool design among corvids suggest cultural variation. Again, apes are the only other animals known to use tools in such a fashion (Clayton and Emery 2005). Nutcrackers and jackdaws were compared in a 2002 study based on geometric rule learning. The corvids, along with a pigeon, had to locate a target between two landmarks, during which distances and landmarks were altered. The nutcrackers resulted in searching more accurately than the jackdaw and pigeon (Jones et al 2002).

A very popular crow scare tactic in the agricultural business is the scarecrow. However, due to the corvid’s quick wit, scarecrows are soon ignored and used as perches. Despite farmers efforts to rid themselves of corvid pests, their attempts have only expanded corvid territories and strengthened their numbers [25]. Recent taxonomy places corvids, based on their evolutionary progess, in the middle of the passerines, despite efforts to promote them to the most advanced of the birds [26].

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Geographic range

Corvids are found worldwide except for the tip of South America and the polar ice caps (Clayton and Emery 2005). Recently the Corvus genus has reentered Australia, resulting in five new species and one new subspecies (see crows).

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Threatened species

Despite the fact that most corvids are not threatened (but are most likely secured by human interaction) a few species are in danger. For example, the ruin of the Southeast Asian rainforests is endangering mixed species bird flocks with members from the Family Corvidae (Lee et al 2005). Also, since scrub is an endangered ecosystem, the Florida Scrub-jays are threatened with extinction (Breiniger, et al 2006).

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Photos-of species, eggs, nests and young

Jackdaw nest[27], Crow Nest [28], Raven nest, eggs, Jay’s, Nutcrackers, Magpies [29], Raven, eggs and nestlings [30], Rook, nest [31], Crows, ravens, rooks, jackdaws, magpies, treepies and nutcrackers pictures [32]

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Songs/Calls:

For all corvid calls.[33]

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Citations

Berg, EC. 2005. Parentage and reproductive success in the white-throated magpie-jay, Calocitta Formosa, a cooperative breeder with female helpers. Animal Behavior 70: 375-385.

Breininger DR, Toland B, Oddy DM, Legare ML. 2006. Landcover characterizations and Florida scrub-jay (Aphelocoma coerulescens) population dynamics. Biological Conservation 128: 169-181.

Bond AB, Kamil AC, Balda RP. 2003. Social complexity and transitive inference in corvids. Animal Behavior 65: 479-487.

Clayton N, Emery N. 2005. Corvid cognition. Current Biology Magazine 15 (3): 80-81.

Everding SE, Jones DN. 2006. Communal roosting in a suburban population of Torresian crows (Corvus orru). Landscape and Urban Planning 74: 21-33.

Gill FB. 2003. Ornithology. 2nd edition. New York: W.H. Freeman and Company. 766 p.

Jones JE, Antoniadis E, Shettleworth SJ, Kamil AC. 2002. A Comparative Study of Geometric Rule Learning by Nutcrackers (Nucifraga colimbiana), Pigeons (Columba livia), and Jackdaws (Corvus monedula). Journal of Comparative Psychology 116 (4): 350-356.

Krushinskii LV, Zorina ZA, Dashevskii BA. 1979. Ability of birds of the Corvidae family to operate by the empirical dimensions of figures. National Library of Medicine 29 (3): 590-7.

Lee TM, Soh MCK, Sodhi N, Koh LP, Lim SLH. 2005. Effects of habitat disturbance on mixed species bird flocks in a tropical sub-montane rainforest. Biological Conservation 122: 193-204.

Marzluff JM, Angell T. 2005. In the Company of Crows and Ravens. New Haven, Connecticut: Yale University Press. 384 p.

Marzluff JM, Neatherlin E. 2006. Corvid response to human settlements and campgrounds: Causes, consequences, and challengers for conversation. Biological Conservation 130: 301-314.

Verhulst S, Salomons HM. 2004. Why flight? Socially dominant jackdaws, Corvus monedula, have low fitness. Animal Behavior 68: 777-783.af:Kraai bg:Вранови cs:Krkavcovití da:Kragefugle de:Rabenvögel eo:Korvo es:Corvidae fr:Corvus it:Corvidae he:עורביים lt:Varniniai paukščiai li:Krejachtege nl:Kraaiachtigen ja:カラス pl:Krukowate pt:Corvidae sk:Krkavcovité sl:Vrani sv:Kråkfåglar zh:鸦科

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