Corvidae


Corvidae is a cosmopolitan family of oscine passerine birds that contains the crows, ravens, rooks, magpies, jackdaws, jays, treepies, choughs, and nutcrackers. In colloquial terms, the crow family is made up of corvids; currently 135 species are included. The genus Corvus containing 47 species makes up over a third of the entire family. The raven is the largest passerine bird.
Corvids display remarkable intelligence for animals of their size, and are among the most intelligent birds thus far studied. Specifically, members of the family have demonstrated self-awareness in mirror tests and tool-making ability, skills which until recently were thought to be possessed only by humans and a few other 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 feet and bills, rictal bristles, and a single moult each year. Corvids are found worldwide, except for the southern tip of South America and the polar ice caps. The majority of the species are found in tropical South and Central America and in southern Asia, with fewer than 10 species each in Africa and Australasia. The genus Corvus has re-entered Australia in relatively recent geological prehistory, with five species and one subspecies there. Several species of raven have reached oceanic islands, and some of these species are now highly threatened with extinction, or have already become extinct.

Systematics, taxonomy, and evolution

The name Corvidae for the family was introduced by the English zoologist William Elford Leach in a guide to the contents of the British Museum published in 1820. Over the years, much disagreement has arisen 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 spread throughout the world from there. Other lineages derived from these ancestors evolved into ecologically diverse, but often Australasian, groups. In the late 1970s and throughout the 1980s, Sibley and Ahlquist united the corvids with other taxa in the Corvida, based on DNA–DNA hybridization. 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 constitute the core group of the Corvoidea, together with their closest relatives. They are also the core group of the Corvida, which includes the related groups, such as Old World orioles and vireos.
Clarification of the interrelationships of the corvids has been achieved based on cladistic analysis of several DNA sequences. The jays and magpies do not 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 of undistinguished lineage, is less clear than previously thought.
The crested jayshrike is traditionally included in the Corvidae, but is not a true member of this family, being closer to the helmetshrikes or shrikes. Likewise, the Hume's ground "jay" is, in fact, a member of the tit family, Paridae. The following tree showing the phylogeny of the crow family is based on a molecular study by Jenna McCullough and collaborators published in 2023.

Fossil record

The earliest corvid fossils date to mid-Miocene Europe, 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
  • Miopica
  • Miocitta
  • Corvidae gen. et sp. indet.
  • Protocitta
  • Corvidae gen. et sp. indet. – probably belongs in an extant genus
  • Henocitta
In addition, there are numerous fossil species of extant genera since the Mio–Pliocene, mainly European Corvus.

Morphology

Corvids are large to very large passerines with a robust build and strong legs; 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 family includes the largest members of the passerine order.
The smallest corvid is the dwarf jay, at and. The largest corvids are the common raven and the thick-billed raven, both of which regularly exceed and.
Species can be identified based on size, shape, and geography; however, some, especially the Australian crows, are best identified by their raucous calls.

Ecology

Corvids occur in most climatic zones. Most are sedentary, and do not migrate significantly. However, during a shortage of food, irruptive migration can occur. When species are migratory, they will form large flocks in the fall and travel south.
One reason for the success of crows, compared to ravens, is their ability to overlap breeding territory. During breeding season, crows were shown to overlap breeding territory six times as much as ravens. This invasion of breeding ranges allowed a related increase in local population density.
Since crows and magpies have benefited and even increased in numbers due to human development, it was suggested that this might cause increased rates of nest predation of smaller bird species, leading to declines. Several studies have shown this concern to be unfounded. One study examined American crows, which had increased in numbers, were a suspect in nest predation of threatened marbled murrelets. However, Steller's jays, which are successful independently 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 did not significantly increase nest predation when compared to other factors, such as habitat destruction. Similarly, a study examining the decline of British songbirds found no link between Eurasian magpie numbers and population changes of 23 songbird species.

Behaviour

Some corvids have strong organization and community groups. Jackdaws, for example, have a strong social hierarchy, and are facultatively colonial during breeding. Providing mutual aid has also been recorded within many of the corvid species.
Young corvids have been known to play and take part in elaborate social games. Documented group games follow "king of the mountain" or "follow the leader" patterns. Other play involves the manipulation, passing, and balancing of sticks. Corvids also take part in other activities, such as sliding down smooth surfaces. These games are understood to play a large role in the adaptive and survival ability of the birds.
Mate selection is quite complex, and accompanied with much social play in the Corvidae. Youngsters of social corvid species undergo a series of tests, including aerobatic feats, before being accepted as a mate 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 birds of prey. Most of the time, these assaults take place as a distraction long enough to allow an opportunity for stealing food.

Food and feeding

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 human food sources. 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 human food sources is contributing to population rises in some corvid species.
Some corvids are predators of other birds. During the wintering months, corvids typically form foraging flocks. However, some crows also eat many agricultural pests, including cutworms, wireworms, grasshoppers, and harmful weeds. 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.

Reproduction

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. 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; jackdaws are known to breed in buildings and in rabbit warrens. 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.
Corvids use several different forms of parental care, including bi-parental care and cooperative breeding. Cooperative breeding takes place when parents are helped in raising their offspring, usually by relatives, but also sometimes by non-related adults. 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.