It is great to see such a longstanding and active thread on plumage
characteristics and terminology. I don't recall that a fine article on the
subject by Claudia Wilds has been recommended in previous postings (and if
it has, forgive the repetition):

Claudia Wilds. 1989. The terminology of plumage and molt. Birding
21:148-153.

As mentioned before, the key article for this subject is Humphrey and
Parkes 1959 (Auk 76:1-31). Wilds bases her descriptions on this article, a
followup by the same authors in 1963 (Auk 80:496-503), and some
publications of Ralph Palmer, cited in her bibliography. Hers is a clear,
readable synthesis.

Hal Opperman
Medina, WA