Corvus (genus)

From Wikipedia, the free encyclopedia

 It has been suggested that Raven be merged into this article or section. (Discuss)
It has been suggested that Crow be merged into this article or section. (Discuss)
This article needs additional citations for verification.
Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (July 2007)
Corvus
Common Raven (Corvus corax)
Common Raven (Corvus corax)
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Aves
Order: Passeriformes
Family: Corvidae
Genus: Corvus
Linnaeus, 1758
Species

See text

The genus Corvus consists of large Passerine birds. They are either black all over, or mainly black with white or grey patches. They range in size from the relatively small jackdaws (still larger than most other passerines) to the very large Common Raven of the Holarctic region and Thick-billed Raven of the highlands of Ethiopia. The 40 or so members of this genus occur on all continents except South America and Antarctica, and on many offshore and oceanic islands.

Members of the group as a whole are known as "True Crows"[1]. Most species are known as "crows", others as "jackdaws", "raven" or "rook"; some of these names reflect relationships within the genus, while others do not.

Contents

[edit] Systematics

The genus was originally described by Linnaeus in his 18th century work Systema Naturae.[2] The name is from the Latin corvus meaning "raven".[3] The type species is the Common Raven (Corvus corax); others named in the same work include the Carrion Crow (C. corone), the Hooded Crow (C. cornix), the Rook (C. frugilegus), and the Jackdaw (C. monedula). The genus was originally broader, as the Magpie was designated C. pica before later being moved into a genus of its own. There are now considered to be at least 42 extant species in this genus, and at least 14 extinct species have been described.

There is no good systematic approach to subdividing the genus at present[citation needed]. Generally, it is assumed that species from a geographical area are more closely related to each other than to other lineages, but this is not necessarily correct. For example, while the Carrion/Collared/House Crow complex is certainly closely related to each other, the situation is not at all clear regarding the Australian/Melanesian species. Furthermore, as many species are similar in appearance, determining actual range and characteristics can be very difficult, such as in Australia where the five (possibly six) species are almost identical in appearance.

The fossil record of crows is rather dense in Europe, but the relationships among most prehistoric species is not clear. Jackdaw-, crow- and raven-sized forms seem to have existed since long ago and crows were regularly hunted by humans up to the Iron Age[citation needed], documenting the evolution of the modern taxa. American crows are not as well documented.[citation needed] They appear to have evolved in central Asia and radiated out into North America, Africa, Europe, and Australia[citation needed].

The latest evidence[4] appears to point towards an Australasian origin for the early family (Corvidae) though the branch that would produce the modern groups such as jays, magpies and large predominantly black Corvus. Crows had left Australasia and were now developing in Asia. Corvus has since re-entered Australia (relatively recently) and produced five species with one recognized sub-species.

A surprisingly high number of species have become extinct after human colonization; the loss of one prehistoric Caribbean crow could also have been related to the last ice age's climate changes.[citation needed]

[edit] List of Species in the genus Corvus

House Crows (Corvus splendens) grooming each other in Kolkata, India
House Crows (Corvus splendens) grooming each other in Kolkata, India

Australian and Melanesian species

New Zealand species

Pacific island species

A Pied Crow (Corvus albus) in flight
A Pied Crow (Corvus albus) in flight

Tropical Asian species

Eurasian and North African species

A Rook (Corvus frugilegus) at Dartmoor, England
A Rook (Corvus frugilegus) at Dartmoor, England

Holarctic species

North and Central American species

Tropical African species

In addition to the prehistoric forms listed above, some extinct chronosubspecies have been described. These are featured under the respective species accounts.

[edit] Behavior

[edit] Calls

Corvus brachyrhynchos call

Call of Corvus brachyrhynchos (American Crow)
Problems listening to the file? See media help.

Crows make a wide variety of calls or vocalizations. Whether the crows' system of communication constitutes a language is a topic of debate and study. Crows have also been observed to respond to calls of other species; this behavior is presumably learned because it varies regionally. Crows' vocalizations are complex and poorly understood. Some of the many vocalizations that crows make are a "caw", usually echoed back and forth between birds, a series of "caws" in discrete units, counting out numbers, a long caw followed by a series of short caws (usually made when a bird takes off from a perch), an echo-like "eh-aw" sound, and more. These vocalizations vary by species, and within each species vary regionally. In many species, the pattern and number of the numerical vocalizations have been observed to change in response to events in the surroundings (i.e. arrival or departure of crows). Crows can hear sound frequencies lower than those that humans can hear, which complicates the study of their vocalizations.

[edit] Intelligence

Hooded Crow searching for food from a punctured garbage bag
Hooded Crow searching for food from a punctured garbage bag

True crows have long been viewed as very intelligent by humans (as seen in Aesop's fable of The Crow and the Pitcher) and they often score very highly on animal intelligence tests, some species even top the avian IQ scale [5]. Crows in the northwestern U.S. Show modest linguistic capabilities and the ability to relay information over great distances, live in complex, hierarchic societies involving hundreds of individuals with various "occupations."[citation needed]

Many of the examples of intelligence involve the various use of tools that the True Crows readily use[6]. For example, hooded crows in Israel have learned to use bread crumbs for fishing bait.[7] Another species, the New Caledonian Crow, has recently been intensively studied because of its ability to manufacture and use its own tools in the day-to-day search for food, including dropping seeds into a heavy trafficked street and waiting for a car to crush them open[8]. On October 5, 2007, researchers from the University of Oxford, England presented data acquired by mounting tiny video cameras on the tails of New Caledonian Crows. It turned out that they use a larger variety of tools than previously known, plucking, smoothing and bending twigs and grass stems to procure a variety of foodstuffs[9].

Crow species in Australia are some of the few animals that have learned how to eat the toxic cane toad. A crow flips the toad over onto its back with its beak and then kills the toad by striking the thin underbelly skin with their beak. This thin skin lacks the poison glands that protect the toads from being eaten from above.[10]

Additionally, it has been suggested that Common Ravens follow wolves or human hunters to kills, effectively using them as "tools," that provide meat. It has even been rumored that they might lead hunters to animals so that they might then eat the leftovers.[11]

[edit] Color and society

 This article may require cleanup to meet Wikipedia's quality standards.
Please improve this article if you can. (March 2008)

[edit] Extra-specific uses of color in crow societies

Many crow species are all black. Most of their natural enemies, the raptors or "falconiformes", soar high above the trees, and hunt primarily on bright, sunny days when contrast between light and shadow is greatest.[citation needed] Crows take advantage of this by maneuvering themselves through the dappled shades of the trees, where their black color renders them effectively invisible to their enemies above[citation needed], in order to set up complex ambush attacks[citation needed]. Fledglings are much duller than adults in appearances of great strategic importance to their societies[citation needed]. It is perhaps here where we find the greatest difference between ravens and crows; ravens tend to soar high in the air as raptors do.[citation needed]

While hawks tend to be the primary daytime predators of crows, their most deadly predators, in many areas, are the owls that hunt by night[citation needed]. Crows also will often mob owls much more fiercely when they find them in daylight than the hawks and other raptors[citation needed]. Frequently crows appear to "play" with hawks, taking turns "counting coup" while escorting the raptor out of their territory. Their attacks on owls, on the other hand, possess a definite serious quality.[citation needed]

[edit] Intra-specific uses of color in crow societies

Even in species characterized by being all black, one will still occasionally find variations, most of which appear to result from varying degrees of albinism, such as:

  • an otherwise all-black crow stunningly contrasted by a full set of brilliant, pure-white primary feathers.
  • complete covering in varying shades of grey (generally tending toward the darker side).
  • blue or red, rather than swarthy eyes (blue being more common than red).
  • Some combination of the above

The treatment of these rare individuals may vary from group to group, even within the same species. For example, one such individual may receive special treatment, attention, or care from the others in its group[citation needed], while another group of the same species might exile such individuals, forcing them to fend for themselves.[citation needed] The reason for such behaviors, and why these behaviors vary as they do, is unknown.

[edit] True Crows and man

Certain species have been considered pests; the Common Raven, Australian Raven and Carrion Crow have all been known to kill weak lambs as well as eating freshly dead corpses probably killed by other means. Rooks have been blamed for eating grain in the UK and Brown-necked Raven for raiding date crops in desert countries.[12]

[edit] See also

[edit] Notes and Citations

  1. ^ Crows
  2. ^ (Latin) Linnaeus, C (1758). Systema naturae per regna tria naturae, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata.. Holmiae. (Laurentii Salvii)., 824. 
  3. ^ Simpson, D.P. (1979). Cassell's Latin Dictionary, 5, London: Cassell Ltd., 883. ISBN 0-304-52257-0. 
  4. ^ Phylogeny and diversification of the largest avian radiation - Barker et al., 10.1073/pnas.0401892101 - Proceedings of the National Academy of Sciences
  5. ^ BBC NEWS | Science/Nature | Crows and jays top bird IQ scale
  6. ^ The Mentality of Crows: Convergent Evolution of Intelligence in Corvids and Apes - Emery and Clayton 306 (5703): 1903 - Science
  7. ^ Animal Intelligence - Birds' Mind: Bait-Fishing Crows
  8. ^ http://en.wikipedia.org/wiki/The_Most_Extreme#Season_1
  9. ^ Discovery News Top Stories : Discovery Channel
  10. ^ Toads fall victim to crows in NT - ABC News (Australian Broadcasting Corporation)
  11. ^ Heinrich 1991
  12. ^ Goodwin 1983

[edit] References

[edit] External links

Wikimedia Commons has media related to: