Leucism is a rare form of an animal having a whitish fur, plumage or skin due to a lack of pigment.
Variability is just a colour difference. In the case of the feral forms of Feral Pigeon’s descended from the Rock Dove of North America it can range from black, blue and white through to red. A whitish Feral Pigeon was noted by museum scientist Kevin Cole feeding along with other doves in the museum garden this December.
Museum ornithologist Phil Whittington comments that in this case it is just a colour variation.