The Carolina Chickadee is very similar to the Black-capped Chickadee. The differences in color and markings are very subtle. It is difficult to tell from these images as they represent only about 5% of the original photo. I shot from such a distance and the bird is sooo small.
It is probable that if I saw both the Black-capped and the Carolina Chickadees side by side, I wouldn't be able to tell them apart. The Carolina has more grey on the chest rather than the buff color of the BCC. There is also a slight difference in the lower edge of the black bib where the one on the BBC is not as smooth as the Carolina.
To be able to distinguish the differences requires a good set of binoculars, photos and a lot of practice. How do I know that this is a Carolina? Well, it was in the South. The Black-capped Chickadee inhabits more northern and western ranges while the Carolina is found in the South. Of course, sometimes birds get confused and cross boundaries, but I think in this case, I have a Carolina Chickadee.