When we look at it, what we find overall is a fairly standard dove-gray bird weighing about 8 oz. Yet, in its vast range there are some obvious differences in its various population. Some show red orbital skin around the eyes rather than the more common dark skin. Some have a dark rump rather than a light one. The shade of blue also varies with some populations being darker than others.
Some groups have youngsters with dark feet in the nest, some with light.
There are also some minor differences in voice and size. Simple, we pick one. In biology, the base line group is the one which is first formally identified and described. In the case of the common pigeon, this is that population of pigeons found in western Europe. Here the common pigeon tends to have orange-red eyes, light rump and a fairly light blue color.