The Long-tailed Duck in the first two photos is a male molting from winter plumage to spring plumage. For example, you can still see some of the black on its breast of winter plumage as it molts to the complete white of spring plumage. The Long-tailed Duck in the third photo is a male that has completely molted to spring plumage.
The Long-tailed Ducks in the fourth and fifth photos are females in spring plumage.