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Corvidae
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Everything about Corvidae totally explained; ;
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Corvidae is a cosmopolitan family of oscine passerine birds that contains the crows, ravens, rooks, jackdaws, jays, magpies, treepies, choughs and nutcrackers. Collectively its members are called corvids and there are over 120 species. The genus Corvus, including the crows and ravens, makes up over a third of the entire family.
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. 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. The genus Corvus has re-entered Australia in relatively recent geological prehistory, with five species and one subspecies there (see crows).
Systematics, taxonomy and evolution
The earliest corvid fossils date to the mid-Miocene, about 17 million years ago; Miocorvus and Miopica may be ancestral crows and some magpie lineage, respectively, or similar to the living forms due to convergent evolution.
Over the years there has been much disagreement on the exact evolutionary relationships of the corvid family and their relatives. What eventually seemed clear was that 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. Sibley and Ahlquist united the corvids with other taxa in the Corvida. The presumed corvid relatives included currawongs, birds of paradise, whipbirds, quail-thrushes, whistlers, monarch flycatchers and drongos, shrikes, vireos and vangas, but current research favors the theory that this grouping is partly artificial. The corvids do constitute the core groups of the Corvoidea together with their closest relatives - apparently birds of paradise, Australian mud-nesters and shrikes - and of the Corvida which should only include the Corvoidea and truly related groups such as orioles and vireos.
Clarification of the interrelationships of the corvids has been achieved based on based on cladistic analysis of several DNA sequences. The jays and magpies don't constitute monophyletic lineages, but rather seem to split up into an American and Old World lineage, and an Holarctic and Oriental lineage, respectively. These are not closely related among each other. The position of the Azure-winged Magpie, which has always been a major enigma, is even more unclear than it was before.
The Crested Jay (Platylophus galericulatus) is traditionally included in the Corvidae, but might not to be a member of this family, possibly closer to the helmet-shrikes (Malaconotidae) or shrikes (Laniidae); it's best considered Corvoidea incertae sedis for the time being. Likewise, the Hume's Ground "Jay" (Pseudopodoces humilis) is in fact a member of the tit family Paridae.
Fossil record
Prehistoric corvid genera are probably mainly of the New World and Old World jays and Holarctic magpies lineages:
Miocorvus (Middle Miocene of Sansan, France)
Miopica (Middle Miocene of SW Ukraine)
Miocitta (Pawnee Creek Late Miocene of Logan County, USA)
Corvidae gen. et sp. indet. (Edson Early Pliocene of Sherman County, USA)
Protocitta (Early Pleistocene of Reddick, USA)
Corvidae gen. et sp. indet. (Early/Middle Pleistocene of Sicily) - probably belongs into extant genus
Henocitta (Arredondo Clay Middle Pleistocene of Williston, USA)
In addition, there are numerous fossil species of extant genera since the Mio-Pliocene, mainly European Corvus. See the genus accounts for more.
Biology
Morphology
Corvids are large to very large passerines with a robust build, strong legs and all species except the Pinyon Jay have nostrils covered by bristle-like feathers. Many corvids of temperate zones have mainly black or blue coloured plumage; however, some are pied black and white, some have a blue-purple iridescence and many tropical species are brightly coloured. The sexes are very similar in color and size. Corvids have strong, stout bills and large wingspans. The larger species are the largest members of the passerine order.
The smallest corvid is the Dwarf Jay (Aphelocoma nana), at 40 g (1.4 oz) and 21.5 cm (8.5 inches). The largest corvids are the Common Raven (Corvus corax) and the Thick-billed Raven (Corvus crassirostris), both of which regularly exceed 1400 grams (3 lbs) and 65 cm (26 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 are predators of other birds. During the wintering months, corvids typically form foraging flocks. 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.
Since crows don't seem to mind human development, it was suggested that the crow population increase would cause increased rates of nest predation. However, Steller's Jays, which are successful independent of human development, are more efficient in plundering small birds' nests than American Crows and Common Ravens. Therefore, the human relationship with crows and ravens didn't increase nest predation by much, especially compared to other factors such as habitat destruction. Some, including the Rook and the Jackdaw, are also communal nesters.
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.
Jackdaws can breed in buildings or in rabbit warrens. 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. Such helpers at the nest in most cooperatively breeding birds are males, while females join other groups.
Social life
Some corvids have strong organization and community groups. Jackdaws, for example, have a strong social hierarchy, and are facultatively colonial during breeding.
Mate selection is also quite complex and accompanied with much social play in the Corvidae. Youngsters of social corvids undergo a series of tests, including aerobatic feats, before being accepted as a mate by the opposite sex. Their intelligence is boosted by the long growing up period of the young. By remaining with the parents, the young have more opportunities to learn necessary skills. Since most corvids are cooperative brooders, their young can learn from different members of the group. A metaanalysis testing how often birds invented new ways to acquire food in the wild found corvids the most innovative birds. A 2004 review suggests their cognitive abilities are on par with those of great apes. Despite structural differences, the brains of corvids and great apes both evolved the ability to make geometrical measurements. Some corvids demonstrate the capacity for imagination, something that only humans were thought to possess. For example, they remember the previous relevant social context, use their own experience of having been a thief to predict the behavior of a pilferer, and determined the safest course to protect the caches from pilferage. Studies to assess like cognitive abilities in apes have been inconclusive.
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.
There are also specific examples of corvid cleverness. One Carrion 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.(External Link ) 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've 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, which 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.
An archetypical scare tactic in the agricultural business is the scarecrow. However, due to the corvids quick wit, scarecrows are actually 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. and the raven banner was the flag of various Viking Age Scandinavian chieftains. He was also attended by Hugin and Munin, two ravens who whispered news into his ears. The Valravn sometimes appears in modern Scandinavian folklore.
The 6th century BC Greek scribe Aesop featured corvids as intelligent antagonists in many fables. Later, in western literature, popularized by American poet Edgar Allan Poe's work "The Raven", the Common Raven becomes a symbol of the main character's descent into madness.
Status and conservation
Unlike many other bird families, corvid fitness and reproduction, especially with many crows, has increased due to human development. The survival and reproductive success of certain crows and ravens is assisted by their close relationship with humans. Also, since its semiarid scrubland habitat is an endangered ecosystem, the Florida Scrub-jay has a small and declining population. A number of island species, which are more vulnerable to introduced species and habitat loss, have been driven to extinction, like the New Zealand Raven, or are threatened, like the Mariana Crow.
In the USA 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 Northwestern and Fish Crows.
Footnotes
Citations
(2005): Parentage and reproductive success in the white-throated magpie-jay, Calocitta formosa, a cooperative breeder with female helpers. Animal Behaviour 70(2): 375-385. (HTML abstract)
|year=2004|id=40320|title=Aphelocoma coerulescens|downloaded=11 May 2006}}
(2006): Landcover characterizations and Florida scrub-jay (Aphelocoma coerulescens) population dynamics. Biological Conservation 128(2): 169-181. PDF fulltext
(2003): Social complexity and transitive inference in corvids. Animal Behaviour 65(3): 479-487. PDF fulltext
(2006): Living with the Trickster: Crows, Ravens, and Human Culture. PLoS Biol 4 (1):e14.
(2005): Corvid cognition. Current Biology 15(3): R80-R81. PDF fulltext
(2004): The Mentality of Crows: Convergent Evolution of Intelligence in Corvids and Apes. Science 306(5703): 1903 - 1907
(2005): Inter-generic relationships of the crows, jays, magpies and allied groups (Aves: Corvidae) based on nucleotide sequence data. J. Avian Biol. 36: 222-234. PDF fulltext
(2003) Ornithology (2nd edition). W.H. Freeman and Company, New York. ISBN 0-7167-2415-4
(1986) Crows of the world. (2nd edition). British Museum of Natural History. ISBN 0-565-00979-6
(2003): Pseudopodoces humilis, a misclassified terrestrial tit (Aves: Paridae) of the Tibetan Plateau: evolutionary consequences of shifting adaptive zones. Ibis 145(2): 185–202. PDF fulltext
(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
(2006): A phylogenetic supertree of oscine passerine birds (Aves: Passeri). Zool. Scripta 35(2): 149–186. (HTML abstract)
(1979): [Abilityof birds of the Corvidae family to operate by the empirical dimensions of figures]. Zhurnal vysshe nervno deiatelnosti imeni IP Pavlova 29(3): 590-597. [Articlein Russian] PMID 112801 (HTML abstract)
(2000): High frequency of extrapair fertilization in a plural breeding bird, the Mexican jay, revealed by DNA microsatellites. Animal Behaviour 60(6): 867-877
(2005): Effects of habitat disturbance on mixed species bird flocks in a tropical sub-montane rainforest. Biological Conservation 122(2): 193-204. (HTML abstract)
(1993): Crows and Jays. Helm. ISBN 1-873403-18-6
(2005): In the Company of Crows and Ravens. Yale University Press, New Haven, Connecticut. ISBN 0-300-10076-0
(2006): Corvid response to human settlements and campgrounds: Causes, consequences, and challenges for conservation. Biological Conservation 130(2): 301-314. (HTML abstract)
(1971): Ecological studies of the Rook Corvus frugilegus L. in northeast Scotland. Dispersion. J. Appl. Ecol. 8: 815-833.
(2000): Bird Families of the World: Corvidae. Created 2000-JAN-30. Retrieved 2007-NOV-10.
([1991]): Phylogeny and Classification of Birds: A Study in Molecular Evolution. Yale University Press, New Haven, CT. ISBN 0-300-04085-7
(2004): Why fight? Socially dominant jackdaws, Corvus monedula, have low fitness. Animal Behaviour 68: 777-783. (HTML abstract)
(1937): The Eared Grebe and other Birds from the Pliocene of Kansas. Condor 39(1): 40. PDF fulltext DjVu fulltextFurther Information
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