Corvidae

The earliest corvid fossils date to mid-Miocene Europe,[15] about 17 million years ago; Miocorvus and Miopica may be ancestral to crows and some of the magpie lineage, respectively, or similar to the living forms due to convergent evolution. The known prehistoric corvid genera appear to be mainly of the New World and Old World jay and Holarctic magpie lineages:

• Miocorvus (Middle Miocene of Sansan, France)
• Miopica (Middle Miocene of SW Ukraine)
• Miocitta (Pawnee Creek Late Miocene of Logan County, US)
• Corvidae gen. et sp. indet. (Edson Early Pliocene of Sherman County, US)[16]
• Protocitta (Early Pleistocene of Reddick, US)
• Corvidae gen. et sp. indet. (Early/Middle Pleistocene of Sicily) - probably belongs in an extant genus
• Henocitta (Arredondo Clay Middle Pleistocene of Williston, US)

In addition, there are numerous fossil species of extant genera since the Mio–Pliocene, mainly European Corvus.[lower-alpha 1]

Corvids are highly opportunistic foragers. Here a jungle crow feeds on a shark carcass.

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.[19]

Some corvids are predators of other birds. During the wintering months, corvids typically form foraging flocks.[2] However, some crows also eat many agricultural pests including cutworms, wireworms, grasshoppers, and harmful weeds.[18] Some corvids will eat carrion, and since they lack a specialized beak for tearing into flesh, they must wait until animals are opened, whether by other predators or as roadkill.

A Canada jay pair feeding their chicks.

Many species of corvid 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 off-territory roosts at night. Some corvids are well-known communal roosters. Some groups of roosting corvids can be very large, with a roost of 65,000 rooks counted in Scotland.[23] 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.[24] Males and females build large nests together in trees or on ledges; jackdaws are known to breed in buildings and in rabbit warrens.[21] The male will also feed the female during incubation.[25] 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.

Corvids use several different forms of parental care, including biparental care and cooperative breeding.[26] Cooperative breeding takes place when parents are helped in raising their offspring, usually by relatives but also sometimes by non-related adults.[27][28] Such helpers at the nest in most cooperatively breeding birds are males, while females join other groups. White-throated magpie-jays are cooperatively breeding corvids where the helpers are mostly female.

The brain-to-body weight ratios of corvid brains are among the largest in birds, equal to that of most great apes and cetaceans, and only slightly lower than a human.[6] Their intelligence is boosted by the long growing period of the young.[29][30][31] By remaining with the parents, the young have more opportunities to learn necessary skills.

When compared to dogs and cats in an experiment testing the ability to seek out food according to three-dimensional clues, corvids out-performed the mammals.[32] A meta-analysis testing how often birds invented new ways to acquire food in the wild found corvids to be the most innovative birds.[33] A 2004 review suggests that their cognitive abilities are on par with those of great apes.[34] Despite structural differences, the brains of corvids and great apes both evolved the ability to make geometrical measurements.

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, and dominance, and to update this information constantly. It might be that social complexity corresponds to their high cognition, as well as contributing to the spread of information between members of the group.[35]

The Eurasian magpie is the only non-mammal species known to be able to recognize itself in a mirror test.[4] Magpies have been observed taking part in elaborate grieving rituals, which have been likened to human funerals, including laying grass wreaths.[36] Marc Bekoff, at the University of Colorado, argues that it shows that they are capable of feeling complex emotions, including grief.[36] Furthermore, carrion crows show a neuronal response that correlates with their perception of a stimulus, which some scientists have argued to be an empirical marker of (avian/corvid) sensory consciousness – the conscious perception of sensory input – in the crows which do not have a cerebral cortex.[37][38] A related study shows that the birds' pallium's neuroarchitecture is reminiscent of the mammalian cortex.[39][40]

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.[41] 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.[42][43] Corvids also move their food around between hiding places to avoid thievery, but only if they have previously been thieves themselves (that is, they remember previous relevant social contexts, use their own experience of having been a thief to predict the behavior of a pilferer, and can determine the safest course to protect their caches from being pilfered). Studies to assess similar cognitive abilities in apes have been inconclusive.[44]

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 California scrub jays, which store perishable foods, not only remember where they stored their food, but for how long. This has been compared to episodic memory, previously thought unique to humans.[3]

New Caledonian crows (Corvus moneduloides) are notable for their highly developed tool fabrication. They make angling tools of twigs and leaves trimmed into hooks, then use the hooks to pull insect larvae from tree holes. Tools are engineered according to task and apparently also to learned preference. Recent studies revealed abilities to solve complicated problems, which suggests high level of innovation of a complex nature.[45][46] Other corvids that have been observed using tools include the American crow, blue jay and green jay. Researchers have discovered that New Caledonian crows don't just use single objects as tools; they can also construct novel compound tools through assemblage of otherwise non-functional elements.[47][48] Diversity in tool design among corvids suggests cultural variation. Again, great apes are the only other animals known to use tools in such a fashion.[3]

Clark's nutcrackers and jackdaws 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. The nutcrackers were more accurate in their searches than the jackdaws and pigeons.[49]

The scarecrow is an archetypal scare tactic in the agricultural business. However, due to corvids' 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.[18]

Contrary to earlier teleological classifications in which they were seen as "highest" songbirds due to their intelligence, current systematics might place corvids, based on their total number of physical characteristics instead of just their brains (which are the most developed of birds), in the lower middle of the passerine evolutionary tree, dependent on which subgroup is chosen as the most derived.[10] As per one observer:

During the 19th century there arose the belief that these were the 'most advanced' birds, based upon the belief that Darwinian evolution brings 'progress'. In such a classification the 'most intelligent' of birds were listed last reflecting their position 'atop the pyramid'. Modern biologists reject the concept of hierarchical 'progress' in evolution [...].[2]

The other major group of highly intelligent birds of the order Psittaciformes (which includes 'true' parrots, cockatoos and New Zealand parrots) is not closely related to corvids.

Proportions of correct responses as a function of species (R: ravens; C: chimpanzees; O: orang-utans)[50]

A study found that four months old ravens can have physical and social cognitive skills similar to that of adult great apes and concludes that "dynamic of the different influences that, during ontogeny, contributes to adult cognition" is required for the study of cognition.[51][50]

Folklore often represents corvids as clever, 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 totems, credited with creating man, and considered responsible for placing the Sun in the sky. [53]

Due to their carrion diet, the 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 Bendigeidfran Blessed raven or the Irish Morrigan, underworld deities that 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 crows, 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 never to be told. Another rhyme is: one for sorrow, two for mirth, three for a funeral, four for a birth, five for heaven, six for hell, and seven for the Devil, his own sel'. Cornish superstition holds that when a lone magpie is encountered, it must be loudly greeted with respect.

Various Germanic peoples highly revered the raven. The major deity Odin was so associated with ravens throughout history that he gained the kenning "raven god"[lower-alpha 2] 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.[54] The Valravn sometimes appears in modern Scandinavian folklore. The Sutton Hoo treasure features stylised corvids with scrolled beaks in the decorative enamel work on the shield and purse lid reflecting their common totemic status to the Anglo-Saxons, whose pre-Christian indigenous beliefs were of the same origin as that of the aforementioned Vikings.

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.

The children's book Mrs. Frisby and the Rats of NIMH and its animated film adaptation features a protagonist crow named Jeremy.

The Hawaiian crow is extinct in the wild as a result of habitat loss and other factors.

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.[19]

Human development provides additional resources by clearing land, creating shrublands rich in berries and insects. When the cleared land naturally replenishes, jays and crows use the young dense trees for nesting sites. Ravens typically use larger trees in denser forest.[19]

Despite the fact that most corvids are not threatened (many even increasing due to human activity) a few species are in danger. For example, the destruction of the Southeast Asian rainforest is endangering mixed-species feeding flocks with members from the family Corvidae.[55] Also, since its semiarid scrubland habitat is an endangered ecosystem, the Florida scrub jay has a small and declining population.[56][57] A number of island species, which are more vulnerable to introduced species and habitat loss, have been driven to extinction, such as the New Zealand raven, or are threatened, like the Mariana crow.

The American crow population of the United States has grown over the years. It is possible that the American crow, due to humans increasing suitable habitat, will cause Northwestern crows and fish crows to decline.[58]