Feral Pigeons (Columba livia domestica), also called City Doves or City Pigeons, are pigeons that are descended from the domestic pigeons that have returned/escaped to the wild.
Research suggests that domestication of pigeons occurred as early as 10,000 years ago.
Pigeons have made contributions of considerable importance to humanity and have served key roles as food, pets, holy animals, post carriers (especially in times of war) and more.
The Domestic Pigeon was originally bred from the wild Rock Dove (Columba livia), which naturally inhabits sea-cliffs and mountains.
Originally found wild in Europe, North Africa, and western Asia, these pigeons find the ledges of buildings to be a substitute for sea cliffs, have become adapted to urban life, and are abundant in towns and cities throughout much of the world.
Rock, Domestic, and Feral Pigeons are all the same species and will readily interbreed. 
Wild Rock Doves are pale grey with two black bars on each wing, whereas Domestic and Feral Pigeons vary in colour and pattern by selective breeding over the years.

St James's Park, London, England, 1 February 2007