Here, showing its right side, the second bird seems, over all, slightly darker than the normal coloration of Eurasian Collared-Doves. Maybe its daddy was a Bonaparte's Gull;o)

Someone mentioned that it may be a hybrid from a Rock Pigeon/ECDO pairing but this bird has no characteristics of such other than the dark coloration. There is no large cere, the size is right on for ECDO and there is no structural or other body color variations from the norm.