Corvidae is a cosmopolitan family of oscine passerine birds containing crows, ravens, ravens, rooks, jackdaws, rooks, jackdaws, magpies, saplings, choughs, and nutcrackers. In common English, they are known as the crow family, or, more technically, the corvids. More than 120 species are described. The genus Corvus, including jackdaws, crows, rooks, and ravens, makes up more than one-third of the entire family.
Corvids show remarkable intelligence for animals of their size and are among the most intelligent birds yet studied. Specifically, members of the family have demonstrated self-awareness in mirror tests (European magpies) and tool-making ability (e.g., crows and rooks), abilities that until recently were thought to be possessed only by humans and some other higher mammals. Their total brain-to-body mass ratio is equal to that of non-human great apes and cetaceans, and only slightly lower than that of humans.
They are medium to large in size, with strong legs and beaks, rictal bristles, and a single molt each year (most passerines molt twice). Corvids are found worldwide except at the tip of South America and on the polar ice caps. Most species are found in tropical South and Central America, southern Asia, and Eurasia, with fewer than 10 species each in Africa and Australasia.
The genus Corvus has again entered Australia in relatively recent geological prehistory, with five species and one subspecies. Several species of crows have reached the oceanic islands, and some of these species are now highly threatened with extinction or are already extinct.
Systematics, taxonomy and evolution of the family corvidae
The family Corvidae was included by the English zoologist William Elford Leach in a guide to the contents of the British Museum published in 1820. Over the years, many disagreements have arisen over the exact evolutionary relationships of the corvid family and its relatives. What finally seemed clear was that corvids are derived from Australasian ancestors and from there spread throughout the world.
Other lineages derived from these ancestors evolved in ecologically diverse, but often Australasian groups. In the late 1970s and throughout the 1980s, Sibley and Ahlquist linked corvids with other Corvid taxa, based on DNA-DNA hybridization.
Presumed relatives of corvids included currawongs, birds of paradise, whipbirds, quail, whistlers, whistlers, monarch flycatchers and drongos, shrikes, vireos and vangas, but current research favors the theory that this group is partly artificial. Corvids form the core of the Corvoidea, along with their closest relatives (birds of paradise, Australian mudskulls, and shrikes). They are also the core of the Corvida, which includes related groups such as the Old World orioles and vireos.
Clarification of corvid interrelationships has been achieved from cladistic analysis of several DNA sequences. Jays and magpies do not constitute monophyletic lineages, but appear to be divided into an American and Old World lineage, and a Holarctic and Oriental lineage, respectively. These are not closely related to each other. The position of the blue-winged magpie, which has always been a major enigma, is even less clear than before.
The bighorn jay (Platylophus galericulatus) is traditionally included in the Corvidae, but may not be a true member of this family, possibly closer to the shrikes (Malaconotidae) or shrikes (Laniidae); for the moment it is best considered Corvidae incertae sedis. Also, Hume’s jay (Pseudopodoces humilis) is in fact a member of the family Paridae. The following tree represents the current knowledge of the phylogeny of the Crow family according to J. Boyd.
The earliest corvid fossils date from the mid-Miocene, about 17 million years ago; Miocorvus and Myopica may be ancestral to crows and some of the magpie lineage, respectively, or similar to living forms due to convergent evolution. The known prehistoric corvid genera appear to be primarily of the New and Old World jay and holarctic magpie lineages:
- Myocorvus (Middle Miocene of Sansan, France).
- Myopica (Middle Miocene of southwestern Ukraine).
- Miocitta (Pawnee Creek Late Miocene of Logan County, US).
- Corvidae gen. et sp. indet. (Edson Early Pliocene of Sherman County, US).
- Protocytta (Early Pleistocene of Reddick, United States).
- Corvidae gen. et sp. indet. (Early PleistoceneOne study examined that American crows, which had increased in numbers, were suspected predators of threatened marbled murrelet nests.
However, Steller’s jays, which are successful independent of human development, are more efficient at plundering small bird nests than American crows and ravens. Thus, human relationships with crows and ravens did not significantly increase nest predation compared to other factors such as habitat destruction. Similarly, a study examining the decline of British songbirds found no relationship between Eurasian magpie numbers and changes in the populations of 23 songbird species.
Some corvids have strong organization and community groups. Jackdaws, for example, have a strong social hierarchy, and are facultatively colonial during breeding. The provision of mutual aid has also been recorded in many of the corvid species.
Young corvids are known to play and engage in elaborate social games. Documented group games follow the patterns of “king of the mountain”, or “follow the leader”. Other games involve manipulation, pacing, and stick swinging. Corvids also engage in other activities, such as sliding across smooth surfaces. These games are understood to play an important role in the birds’ ability to adapt and survive.
Mate selection is quite complex and is accompanied by much social play in Corvidae. Young of social corvid species undergo a series of tests, including acrobatic feats, before being accepted as mates by the opposite sex.
Some corvids can be aggressive. Blue jays, for example, are well known to attack anything that threatens their nest. Crows have been known to attack dogs, cats, ravens, and raptors. Most often these assaults occur as a distraction long enough to allow an opportunity to steal food.
Diet and feeding
The natural diet of many species of corvids is omnivorous, consisting of invertebrates, chickens, small mammals, berries, fruits, seeds and carrion. However, some corvids, especially crows, have adapted well to human conditions and have come to rely on anthropogenic foods. (See Article: Phoenicurus Phoenicurus)
In a U.S. study of American crows, common ravens, and Steller’s jays just around human camps and settlements, the crows appeared to have the most varied diet of all, as the same crows cough up anthropogenic foods such as bread, spaghetti, French fries, dog food, sandwiches, and cattle feed. The increase in available anthropogenic food sources is contributing to the population increase of some corvid species.
Some corvids are predators of other birds. During the winter months, corvids often form foraging flocks. However, some crows also eat many agricultural pests, including cutworms, wireworms, grasshoppers, and noxious weeds. Some corvids eat carrion, and because they lack a specialized beak for tearing meat, they must wait until the animals are cut open, either by other predators or as roadkill.
Many species of corvids are territorial, protecting territories throughout the year or simply during the breeding season. In some cases, territories may only be guarded during the day, with the pair joining roosts outside the territory at night. Some corvids are known communal roosters.Some roosting corvid groups can be very large, with a roost of 65,000 rooks counted in Scotland. Some, including the rook and jackdaw, are also communal roosts.
The pair bond in corvids is extremely strong and even lifelong in some species. This monogamous lifestyle, however, may 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.
Nests are constructed of a mass of bulky branches covered with grass and bark. Corvids can lay between 3 and 10 eggs, usually between 4 and 7. The eggs are usually greenish in color with brown spots. Once hatched, the young remain in the nests for up to 6-10 weeks, depending on the species. Corvids provide biparental care.
Jackdaws may breed in buildings or in rabbit burrows. The white-throated magpie is a cooperative breeding corvid species in which the helpers are mostly females. Cooperative breeding occurs when there are additional adults to help raise the nestlings. These helpers in the nest of most cooperatively breeding birds are males, while females join other groups.
The brain-to-body weight ratio of corvid brains is one of the largest among birds, equal to that of most great apes and cetaceans, and only slightly lower than that of humans. Their intelligence is enhanced by the long growing period of the young. By staying with the parents, the young have more opportunities to learn the necessary skills.
When compared to dogs and cats in an experiment testing the ability to forage according to three-dimensional cues, corvids outperformed mammals. A meta-analysis assessing the frequency with which birds invented new ways of acquiring food in the wild found that corvids were the most innovative birds. A 2004 review suggests that their cognitive abilities are on par with those of great apes. Despite structural differences, the brains of corvids and great apes evolved the ability to perform geometric measurements.
The ingenuity of corvids can be seen through their abilities in feeding, memorization, tool use, 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 feeding of other members over time. Members must also be able to distinguish between sex, age, reproductive status, and dominance, and to update this information constantly. It may be that social complexity corresponds to their high cognition.
The Eurasian magpie is the only known non-mammalian species that can recognize itself in a mirror test. Magpies have been observed engaging in elaborate mourning rituals, which have been compared to human funerals, including the laying of grass wreaths. Marc Bekoff of the University of Colorado argues that this shows they are capable of feeling complex emotions, including grief.
There are also specific examples of horny cunning. A scavenger crow was documented to crack nuts by placing them in a crosswalk, letting 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 the lids off garbage cans while their mates collected food.
Members of the corvid family are known to watch other birds, remember where they hide their food, and then return when the owner leaves. Corvids also move their food from one hiding place to another to avoid theft, but only if they themselves have previously been thieves (i.e., they remember previous relevant social contexts, use their own experience of having been thieves to predict a thief’s behavior, and can determine the safest course to protect their hiding places from being stolen). Studies to assess similar cognitive abilities in apes have been inconclusive.
The ability to hide food requires very precise spatial memories. Corvids have been recorded to remember the hiding of their food up to nine months later. It is suggested that vertical landmarks (such as trees) are used for location recall. There has also been evidence that California scrub jays, which store perishable foods, not only remember where they store their food, but for how long. This has been compared to episodic memory, which was previously thought to be exclusive to humans. (See Article: Scavenging Birds)
New Caledonian crows (Corvus moneduloides) are noted for their highly developed tool making. They make fishing tools from twigs and leaves trimmed into hooks, and then use the hooks to pull insect larvae out of tree holes. The tools are designed according to the task and apparently also to learned preferences.
Recent studies revealed the ability to solve complicated problems, suggesting a high level of innovation of a complex nature. Other corvids that have been observed using tools include the American crow, blue jay, and green jay. The diversity in tool design among corvids suggests cultural variation. Again, great apes are the only other animals known to use tools in this way.
Clark’s nutcrackers and rooks were compared in a 2002 study based on geometric rule learning. The corvids, along with a domestic pigeon, had to locate a target between two landmarks, while distances and landmarks were altered. Nutcrackers were more accurate in their searches than jackdaws and pigeons.
The scarecrow is an archetypal scare tactic in the agricultural business. However, due to the quick wit of corvids, scarecrows are soon ignored and used as perches. Despite farmers’ efforts to get rid of corvid pests, their attempts have only expanded the corvids’ territories and strengthened their numbers.
Contrary to previous teleological classifications in which they were regarded as “superior” songbirds because of their intelligence, current systematics could place corvids, based on their total number of physical characteristics and not just their brain (which is the most developed of the birds), in the lower center of the passerine evolutionary tree, depending on which subgroup is chosen as the most derived. According to one observer:
During the 19th century, the belief arose that these were the “most advanced” birds, based on the belief that Darwinian evolution brings “progress”. In such a classification, the “most intelligent” birds were listed last, reflecting their position “at the top of the pyramid.” Modern biologists reject the concept of hierarchical “progress” in evolution.
The other large group of highly intelligent birds in the order Psittaciformes (which includes “true” parrots, cockatoos, and New Zealand parrots) is not closely related to the corvids.
Corvids are reservoirs (carriers) of West Nile virus in the United States. They are infected by mosquitoes (the vectors), mainly Culex species. Crows and ravens die quickly from this disease, so their deaths are an early warning system when West Nile virus reaches an area (as are the deaths of horses and other bird species). One of the first signs that West Nile virus arrived in the United States in 1999 was the death of crows in New York.
Relationship with humans
Several different corvids, particularly crows, have occasionally served as pets, although they are not able to talk as easily as parrots and do not like to be caged.
Role in myth and culture
Folklore often depicts corvids as intelligent and even mystical animals. Some Native Americans, such as the Haida, believed that a raven created the earth and despite being a trickster spirit, ravens were popular on totem poles, credited with creating man and considered responsible for placing the Sun in the sky.
Because of their scavenging diet, Celtic peoples strongly associated corvids with war, death and the battlefield – their great intelligence meant that they were often considered messengers, or manifestations of the gods such as the blessed raven of Bendigeidfran or the Irish Morrigan, deities of the underworld who may be related to the later Arthurian fisher king. The Welsh dream of Rhonabwy illustrates well the association of ravens with war.
In many parts of Britain, gatherings of ravens, or more often magpies, are counted using the divination rhyme: one for sorrow, two for joy, three for a girl, four for a boy, five for silver six for gold, seven for a secret you never know. Cornish superstition holds that when a solitary magpie is found, it should be received with respect.
Several Germanic peoples greatly revered the raven. The major deity, Odin, was so closely associated with ravens throughout history that he obtained the kenning “raven god” and the raven banner was the banner of several Scandinavian chieftains of the Viking Age. Also in attendance were Hugin and Munin, two ravens who whispered news in his ear The Valravn sometimes appears in modern Scandinavian folklore. The Sutton Hoo hoard features stylized corvids with coiled beaks in the decorative enamel of the shield and on the purse lid, reflecting their totemic status common to the Anglo-Saxons, whose indigenous pre-Christian beliefs were of the same origin as those of the Vikings mentioned above.
The Greek scribe of the 6th century BC. Aesop presented corvids as intelligent antagonists in many fables. Later, in Western literature, which was popularized by American poet Edgar Allan Poe’s “The Raven,” the common raven becomes a symbol of the main character’s descent into madness. In the book Mrs. Frisby and the Rats of NIMH and its film adaptation, a raven named Jeremy appears.
Condition and conservation
Unlike many other bird families, the fitness and reproduction of corvids, especially with many crows, has increased due to human development. The survival and reproductive success of certain crows and ravens are favored by their close relationship with humans.
Human development provides additional resources by clearing land, creating thickets rich in berries and insects. When cleared land is naturally replenished, jays and crows use the young, dense trees for nesting. Crows often use larger trees in denser forests.
Although most corvids are not threatened (many are even increasing due to human activity), some species are endangered. For example, the destruction of rainforests in Southeast Asia is endangering the feeding of mixed-species flocks with members of the family Corvidae. (See Article: Larus Canus)
In addition, because its semi-arid scrub habitat is an endangered ecosystem, the Florida scrub jay has a small and declining population. Several island species, which are more vulnerable to introduced species and habitat loss, have been forced into extinction, such as the New Zealand crow, or are threatened, such as the Mariana crow.
The American crow population in the United States has grown over the years. It is possible that the American crow, because humans are increasing their suitable habitat, will cause Northwest crows and fish crows to decline.