Here is an interesting photo of an American Robin that was in my yard with other "normal dark head" Robins. He remained just long enough for me to take a couple of photos.
Leucism is an abnormal plumage condition caused by a genetic mutation that prevents pigment, particularly melanin from being properly deposited on a bird’s feathers.
As a result, the birds do not have the normal, classic plumage colors listed in field guides and instead the plumages have several color changes, including:
White patches where the bird should not have any.