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Corvidae is a family of oscine passerine birds that contains the crows, ravens, rooks, jackdaws, jays, magpies, treepies and nutcrackers (Clayton and Emery 2005, ). Collectively its members are called corvids and there are over 120 species.
They are medium to large birds with strong feet and bills, rictal bristles and a single moult each year (most passerines moult twice).
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 re-entered Australia, resulting in five new species and one new subspecies (see crows). The majority of the species are found in tropical South and Central America, southern Asia and Eurasia, with fewer than 10 species each in Africa, Australasia and North America
Systematics, taxonomy and evolution
The earliest corvid fossils date to the mid-Miocene (about 17 MYA) . 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 is onomatopoetic, from their raucous “croaking” calls . Corvids are derived from Australasian ancestors and from there, spread throughout the world. Other lineages derived from these ancestors evolved into ecologically diverse, but often Australasian groups. Over the years there has been much disagreement on the exact evolutionary relationships of the corvid family and their relatives. Sibley and Ahlquist have united the corvids with other taxa in the Corvida, but current research favors the theory that this grouping is partly artificial.
Clarification of the interrelationships of the corvids has been researched by Ericson et al. (2005), based on comparison of several DNA sequences. The Crested Jay (Platylophus galericulatus) is traditionally included in the Corvidae, but seems not to be a member of this family. Likewise, the Hume's Ground "Jay" (Pseudopodoces humilis) is in fact a member of the family (Paridae) (titmice). The jays and magpies do not constitute monophyletic lineages, but rather seem to split up in a American and Old World, and a Holarctic and a Oriental lineage, respectively, which are not closely related inter se. The position of the Azure-winged Magpie, which has always been a major enigma, is even more unclear than it was before.
- Old World and
- Stresemann's Bush Crow, Zavattariornis stresemanni
- Azure-winged Magpie, Cyanopica cyana
- New World
- Prehistoric corvid genera (probably mainly New World
and Old World Jays and Holarctic Magpies)
- Henocitta (Arredondo Early Pleistocene of Williston, USA)
- Protocitta (Early Pleistocene of Reddick, USA)
- Corvidae gen. et sp. indet. (Sicily) - probably belongs into extant genus.
In addition, there are numerous fossil species of extant genera (mainly European Corvus). See the genus accounts for more.
Typical size and appearance
Corvids have feathered rounded nostrils, strong tails and wings and similar sexes. Many corvids of temperate zones are mainly black or blue; however, some are pied black and white, some have a blue-purple iridescence and many tropical species are highly coloured. Corvids have strong, stout bills, large wingspans and are between 23 and 71 cm long. 
Members of the genus Corvus are the largest members of the passerine order reaching 50-71 cm (20-27 inches). Species can be identified based on size, shape and geography; however, some, especially the Australian crows, are best identified by their raucous calls. 
Some corvids have 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. 
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 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).
Some corvids can be 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 as a distraction long enough to allow the crow to steal food.
Food and foraging habits
The natural diet of many corvid species is omnivorous, consisting of invertebrates, nestlings, small mammals, berries, fruits, seeds, and carrion. However, some corvids, especially the crows, have adapted well to human conditions and have come to rely on anthropogenic foods. In a US study of American Crows, Common Ravens and Steller's Jays around campgrounds and human settlements, the crows appeared to have the most diverse diet of all, taking 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).
Some corvids are predators of other birds. During the wintering months, corvids typically form foraging flocks . However, some crows also eat many agricultural pests including cutworms, wireworms, grasshoppers and harmful weeds . Some corvid will eat carrion, and since they lack a specialized beak for tearing into flesh, they must wait until animals are opened by other predators or as roadkill.
Corvids occur in most climatic zones. Most are sedentary and do not migrate significantly. However, during a shortage of food, eruptive migration can occur . When species are migratory, they will form large flocks in the fall (around August) and travel south .
Some corvids 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 . The nests are constructed of a mass of bulky twigs lined with grass and bark. Corvids can lay between 3 and 10 eggs, typically ranging between 4 and 7. The eggs are usually greenish in colour 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 .
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.
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).
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).
Since some corvids, especially in the temperate Northern Hemisphere have black feathers and eat carrion, humans have long associated members of Corvidae with death and extreme injustice (Marzluff and Angell 2005). Throughout history, corvids have been perceived as dark messengers, bearing ill will and other demonic associations. This dark connection is reflected by the literary terms coined to describe groups of crows (a murder), ravens (unkindness, constable or conspiracy), and jays (scold). .
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 heroes 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 mediaeval times the raven stood for virility. Legends report that a raven’s favourite food is dead animals, and that they sometimes hunt with wolves . For more myths and legends see crow and raven pages.
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 . 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 that of all other bird species . When compared to other carnivorous mammals (specifically dogs and cats) in one laboratory test, corvid birds outshone 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 .
The corvid ingenuity is represented through their feeding skills, memorization abilities, use of tools, and group behaviour. 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 passing 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. 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 behaviour 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. 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 . Recent taxonomy places corvids, based on their evolutionary progress, in the middle of the passerines, despite efforts to promote them to the most advanced of the birds .
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 destruction of the Southeast Asian rainforests is endangering mixed-species feeding 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).
For all corvid calls.
- Berg, Elena C. (2005): Parentage and reproductive success in the white-throated magpie-jay, Calocitta formosa, a cooperative breeder with female helpers. Animal Behavior 70(2): 375-385. DOI:10.1016/j.anbehav.2004.11.008 (HTML abstract)
- Breininger, D. R.; Toland, B.; Oddy, D. M. & Legare, M. L. (2006): Landcover characterizations and Florida scrub-jay (Aphelocoma coerulescens) population dynamics. Biological Conservation 128: 169-181. PDF fulltext
- Bond, Alan B.; Kamil, Alan C. & Balda, Russell P. (2003): Social complexity and transitive inference in corvids. Animal Behaviour 65(3): 479-487. DOI:10.1006/anbe.2003.2101 (HTML abstract)
- Clayton, Nicola & Emery, Nathan (2005): Corvid cognition. Current Biology 15(3): R80-R81. PDF fulltext
- Ericson, Per G. P.; Jansén, Anna-Lee; Johansson, Ulf S. & Ekman, Jan (2005): Inter-generic relationships of the crows, jays, magpies and allied groups (Aves: Corvidae) based on nucleotide sequence data. Journal of Avian Biology 36: 222-234. PDF fulltext
- Everding, Susanne E. & Jones, Darryl N. (2006): Communal roosting in a suburban population of Torresian crows (Corvus orru). Landscape and Urban Planning 74(1): 21-33. PDF fulltext
- Gill, F. B. (2003) Ornithology (2nd edition). W.H. Freeman and Company, New York. ISBN 0716724154
- Jones, Juli E,; Antoniadis, Elena; Shettleworth, Sara J. & Kamil, Alan C. (2002): A Comparative Study of Geometric Rule Learning by Nutcrackers (Nucifraga columbiana), Pigeons (Columba livia), and Jackdaws (Corvus monedula). Journal of Comparative Psychology 116(4): 350-356. HTML abstract
- Krushinskii, L. V.; Zorina, Z. A. & Dashevskii, B. A. (1979): [Ability of birds of the Corvidae family to operate by the empirical dimensions of figures]. Zhurnal vysshe nervno deiatelnosti imeni IP Pavlova 29(3): 590-597. [Article in Russian]
- Lee, T. M.; Soh, M. C. K.; Sodhi, N.; Koh, L. P. & Lim, S. L. H. (2005): Effects of habitat disturbance on mixed species bird flocks in a tropical sub-montane rainforest. Biological Conservation 122(2): 193-204. DOI:10.1016/j.biocon.2004.07.005 (HTML abstract)
- Marzluff, John M. & Angell, T. (2005): In the Company of Crows and Ravens. Yale University Press, New Haven, Connecticut. ISBN 0300100760
- Marzluff, John M. & Neatherlin, Eric (2006): Corvid response to human settlements and campgrounds: Causes, consequences, and challengers for conversation. Biological Conservation 130(2): 301-314. DOI:10.1016/j.biocon.2005.12.026 (HTML abstract)
- Verhulst, Sion & Salomons, H. Martijn (2004): Why fight? Socially dominant jackdaws, Corvus monedula, have low fitness. Animal Behaviour 68: 777-783. DOI:10.1016/j.anbehav.2003.12.020 (HTML abstract)
- Corvidae videos on the Internet Bird Collection