Crows
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The true crows are in the genus Corvus. They are large passerine birds. All temperate continents (except South America) and several offshore and oceanic islands (including Hawai‘i) have representatives of the 40 or so members of this genus.
Crows in the genus Corvus appear to have evolved in central Asia and radiated out into North America, Africa, Europe, and Australia.
The latest evidence 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.
They range in size from the relatively small pigeon-sized jackdaws (Eurasian and Daurian) to the Common Raven of the Holarctic region and Thick-billed Raven of the highlands of Ethiopia.
In literary and fanciful usage, the collective noun for a group of crows is a murder. However, in practice most people, and especially scientists, use the more generic term flock.
Systematics
There is no good systematic approach to the genus at present. Generally, it is assumed that the 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.
The Neogene 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, documenting the evolution of the modern taxa. American crows are not as well-documented.
A surprisingly high number of species have gone extinct after human colonization; the loss of one prehistoric Caribbean crow could also have been related to the last ice age's climate changes.
Species
Australian and Melanesian species
- Australian Raven C. coronoides
- Forest Raven C. tasmanicus
- Relict Raven C. (t.) boreus
- Little Crow C. bennetti
- Little Raven C. mellori
- Torresian Crow C. orru
- New Caledonian Crow C. moneduloides
- Long-billed Crow C. validus
- White-billed Crow C. woodfordi
- Bougainville Crow C. meeki
- Brown-headed Crow C. fuscicapillus
- Grey Crow C. tristis
- New Ireland Crow, Corvus sp. (prehistoric)
New Zealand species
- Chatham Islands Raven, C. moriorum (prehistoric)
- New Zealand Raven, C. antipodum (prehistoric)
Pacific island species
- Mariana Crow, C. kubaryi
- Hawaiian Crow or ‘Alala C. hawaiiensis (extinct in the wild, formerly C. tropicus)
- High-billed Crow, C. impluviatus (prehistoric)
- Robust Crow, C. viriosus (prehistoric)
Tropical Asian species
- Slender-billed Crow C. enca
- Piping Crow C. typicus
- Banggai Crow C. unicolor (possibly extinct)
- Flores Crow C. florensis
- Collared Crow C. torquatus
- Daurian Jackdaw C. dauricus
- House Crow C. splendens
- Large-billed Crow C. macrorhynchos
- Jungle Crow C. (m.) levaillantii
Eurasian and North African species
- Brown-necked Raven C. ruficollis
- Fan-tailed Raven C. rhipidurus
- Jackdaw C. monedula
- Rook C. frugilegus
- Hooded Crow C. cornix
- Mesopotamian Crow, C. (c.) capellanus
- Carrion Crow C. corone
- Corvus larteti (fossil: Late Miocene of France)
- Corvus antecorax (fossil: Early - Late Pleistocene of Europe; may be subspecies of Corvus corax
- Corvus betfianus (fossil)
- Corvus praecorax (fossil)
- Corvus simionescui (fossil)
- Corvus pliocaenus (fossil)
- Corvus fossilis (fossil)
- Corvus moravicus (fossil)
- ?Corvus hungaricus (fossil)
Holarctic species
- Common Raven C. corax
- Pied Raven, C. c. varius morpha leucophaeus (an extinct color variant)
North and Central American species
- American Crow C. brachyrhynchos
- Chihuahuan Raven C. cryptoleucus
- Fish Crow C. ossifragus
- Northwestern Crow C. caurinus
- Tamaulipas Crow C. imparatus
- Sinaloan Crow C. sinaloae
- Jamaican Crow C. jamaicensis
- White-necked Crow C. leucognaphalus
- Palm Crow C. palmarum
- Cuban Crow C. nasicus
- Puerto Rican Crow C. pumilis (prehistoric; possibly a subspecies of C. nasicus/palmarum)
- Corvus galushai (fossil: Big Sandy Late Miocene of Wickieup, USA)
- Corvus neomexicanus (fossil: Late Pleistocene of Dry Cave, USA)
Tropical African species
- Cape Crow C. capensis
- Pied Crow C. albus
- Somali Crow or Dwarf Raven C. edithae
- Thick-billed Raven C. crassirostris
- White-necked Raven C. albicollis
In addition to the prehistoric forms listed above, some extinct chronosubspecies have been described. These are featured under the respective species accounts.
Calls
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.
Intelligence
As a group, the crows show remarkable examples of intelligence. They top the avian IQ scale. Crows and ravens often score very highly on intelligence tests. Crows in the northwestern U.S. (a blend of Corvus brachyrhynchos and Corvus caurinus) 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", and have an intense rivalry with the area's less socially advanced ravens. One 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. Wild hooded crows in Israel have learned to use bread crumbs for bait-fishing. Crows will engage in a kind of midair jousting, or air-"chicken" to establish pecking order.
Color and society
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. 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, in order to set up complex ambush attacks. Thus, their black coloring is of great strategic importance to their societies. 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, and like raptors, are usually the target of ambushes by crows. Crows do not appear to perceive ravens as their own kind, but instead treat them as raptors.
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, preying upon crows sleeping helplessly in their roosts. Presumably their dark color is particularly helpful in blending into nighttime shadows. Crows also will often mob owls much more fiercely when they find them in daylight than they do hawks and other raptors. 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.
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, while another group of the same species might exile such individuals, forcing them to fend for themselves. The reason for such behaviors, and why these behaviors vary as they do, has yet to be studied.
Copyright: Wikipedia. This article is licensed under the GNU Free Documentation License. It uses material from Wikipedia.org
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- Carrion/Hooded Crow (Corvus corone)
- Hooded Crows: Jon Loman's Research
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