Today's pics were taken yesterday and show some moult in black skimmers. I assume that this first bird is a bird that has nearly completed the moult out of juvie plumage and into adult winter:
This is an adult in the process of moulting into winter plumage, you can see there are still retained brownish-black feathers in the wings. The bird is only starting to form the white nape that's the hallmark of winter plumage.
This bird is still in breeding plumage with a completely black nape. I'm a little surprised that it has glossy black feathers with the fully black nape since the previous bird moulting out of breeding plumage had a lot of faded feathers in the wing. In the field my assumption was that it had completed the moult, not realizing that winter birds have white napes. I think this is a female, I also didn't realize until I later cracked open Sibley, that females are smaller with less hefty bills. I remembered seeing one smaller all black-backed bird in the group, I think this is it. Perhaps there's some difference in the sexes in the moult timing or maybe differences in how fast the feathers wear depending on parental duties.