I've been thinking that I've been a little sloppy with my terminology in how I've been reporting 1st year gulls lately, I've been calling them all juveniles (juvies). Juvenile plumage really refers to the initial full plumage that a bird leaves the nest in. In most species of bird this is a briefly held plumage and either moult or wear quickly gives a new appearance (i.e. the spotted newly fledged robins quickly become red-breasted). Gulls, however, hold that juvenile plumage longer. But, they soon start moulting into what Olsen and Larsson term 1st winter plumage, undergoing what Howell and Dunn refer to as pre-alternate 1 (PA1) moult.
Here's a true juvie herring gull, photographed in August. Note the wholly brown plumage:
Next is a bird from September. It still looks mostly like the first bird, but note how a few scapulars are brown-black with gray edges rather than brown with white notching. This is part of the onset of PA1 moult:
I slacked off acquiring herring gull moult pics in fall, so we skip on to where we are now. This bird is from yesterday in New Buffalo. Note that the back is mainly formed by the blackish scapulars with white edges. The bird has also moulted much of its body such that it is grayer overall and has a whiter head (the bill is also well on its way to becoming bi-colored).
The last 2 birds are what would generically be called 1st winter plumage. Herring gulls, like most large white-headed gulls, suspend their PA1 moult during the tough times of winter, reserving calories for things like not starving to death rather than growing new feathers. Herring gulls are fairly variable in their moult, some apparently do very little moult in their first fall. This is also regional as well; if you look at the pics in Howell and Webb of birds on the west coast they will be far less advanced in their moult in photos taken 2 months later than these are. Our midwestern birds apparently follow more of an east-coast schedule. Also to add to the confusion, once they re-start PA1 moult in the spring, they don't actually reach an alternate plumage, they just keep right on moulting such that PA1 apparently runs into PB2 (pre-basic 2nd year moult) more or less continuously. But we'll leave figuring that out for next year. Howell and Webb captions the current birds 1st cycle in "post-juvenal molt" in his pics, but describe them as undergoing PA1 moult in the text. Olsen and Larsson would call them 1st winter.
And I haven't forgotten my threat to do another white-cheeked goose post!