A comprehensive genetic analysis of all the recognised species and many subspecies of the cosmopolitan crow genus Corvus has been performed with some surprising results.
The monophyly of the genus – that is, its evolutionary source in a close common ancestor – was confirmed and an origin in the Palearctic in the middle of the Miocene geological time period somewhere between 11 and 17.5 million years ago was strongly though imprecisely suggested by the research team's results; Corvus then spread to its current range on all continents except South America and Antarctica, colonising North America and the Caribbean first.
Eight discrete clades – closely related clusters of species again derived from a common ancestor – were identified, with Daurian and Western Jackdaw (C dauuricus and C monedula) being the closest to the genus's original form (though these are split into another genus Coloeus by some authorities, including the International Ornithological Committee, with two partially West Indian clades also including the North American Fish (C ossifragus) and Tamualipas (C imparatus) Crows.
Rook (C frugilegus) and Hawaiian Crow (C hawaiiensis) share a common ancestor in a fourth clade, implying northern Pacific colonisation from the west, while all the African species (bar Cape Crow C capensis) and Raven (C corax) cluster in a fifth. A sixth wide-ranging Holarctic clade contains the familiar Carrion (C corone) and Hooded (C cornix) Crows and American Crow (C brachyrhynchos) in two separated branches, while a seventh contains all the Australasian species except the unresolved Flores Crow (C florensis). A final grouping is centred upon the widespread and very differentiated Large-billed Crow (C macrorhynchos) of south-eastern Asia, as well as the pernicious House Crow (C splendens).
The paper demonstrates a deep division in Australian Raven C coronoides between western (C coronoides perplexus) and eastern (C coronoides coronoides) populations, and the authors suggest both are split to species level. Haring et al (2007) proposed a similar split in Rook between western (C f frugilegus) and eastern (C f pastinator) birds, and this is supported by the present work. The latter also noted a possible division between eastern and western Carrion Crow taxa (C corone corone and C corone orientalis , further supported by data from Haring et al (2012), but no eastern birds were sampled for the new paper to confirm this.
The authors suggest that additional sampling will be needed in the case of the widespread but fragmented Large-billed, Slender-billed (C enca) and Torresian (C orru) Crows, in order to detect the extent of the likely partially undetected variation among these species, which already have up to nine, seven and three subspecies recognised, respectively.
Another purpose of the paper was to explain the evolution of tool use among members of the genus, but the researchers discovered that such innovations were widespread among the species sampled, pointing to such behaviour already existing in their basal ancestor.
Jønsson, K A, Fabre, P-H and Irestedt, M. 2012. Brains, tools, innovation and biogeography in crows and ravens. BMC Evolutionary Biology 12: 72 doi:10.1186/1471-2148-12-72.