A long time ago now I did a couple of blogs on moult in waders. Part 1 dealt with the basics of wear and moult, and looked at this process in an adult Curlew Sandpiper.
Part 2 looked at juveniles. It included the comment: " By this time, the bird is ready to undergo its first moult - into '1st winter plumage'. But that will have to wait for another post!" Well, it's been a long wait, but I realize that I did do some more pictures showing the progression from juvenile to first winter plumage which I never published, so here they are - not the complete set...watch this space (but don't hold your breath!)
Animated version here.
Up to this point the bird is still in complete juvenile plumage; the differences in appearance can be explained by the gradual wearing away of pale feather edges, which makes the bird's overall appearance plainer and darker.
In this series of pictures, two processes are going on. Wear is still occurring to the old juvenile feathers, making them duller and the patterning on them less distinct. At the same time, these old feathers are gradually being moulted or replaced by new, fresh non-breeding plumage feathers. On Curlew Sandpiper, these are generally plain and grey.The head and body feathers (including the mantle, scapulars and underparts) are completely replaced, and a variable number of wing feathers are as well.
Some of the changes are rather subtle, and might be more easily seen in the animated version here.
By November, first-winter birds look very similar to adults in non-breeding plumage (I haven't completed the sequence yet!), but they can usually be aged by the presence of a few old juvenile feathers (coverts) which retain their distinctive pattern, albeit much faded. Even where juvenile feathers lack distinctive patterning, they can often be differentiated from adult non-breeding feathers by their more ragged, heavily worn appearance. Moult proceeds assymetrically, so it's always advisable to get a good look at both sides of a bird as often, the 'give-away' juvenile feather may only be present on one wing.