American Robin variations

Every winter here in the Willamette Valley there is an influx of American Robins, probably moving in from the north or from higher elevations, or both. Presumably it's easier for a Robin to find food here in our relatively warmer and wetter climate than where they come from. And every year I notice a lot of variation in the plumage details when there are several together on the same lawn or field. I'm thinking of starting a collection of Robin photos, so these two, which I saw today, are good examples to begin with. The one above has only a small amount of white on the face and throat; a broken eyering and a small mark in front of the eye. The one below, in addition to a browner head, almost has a "supercillium" or eyebrow, and a more pronounced broken eyering. I wonder if these differences have any significance beyond differences of age and sex.