A genetic survey of corvid museum specimens has revealed that crows readily switched plumage colours during their evolution and suggests new cryptic splits in the main groups. Among the most interesting results is the suggestion that the pied plumage of the currently split Hooded Crow is actually a stable and geographically contained colour morph rather than an indication of species status, indiciating the species' potential to be re-lumped at some stage.
Black-and-white and black-and-grey plumages seem to have evolved several times in the genus Corvus, which also has the ability to genetically revert to black once more.
Pelstra, J. 2013. The Genetics of Speciation and Colouration in Carrion and Hooded Crows. Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 1088, Uppsala University, Finland.