When we learned our native tongue we grew into it slowly. We spoke it at home. We slowly added vocabulary through primary school. There comes a point in middle school, though, when we are finally taught the parts of speech--nouns and verbs, how to diagram a sentence, etc. At the time we thought this unnecessary--we already knew how to read and speak--so what's the point? Learning the parts of speech and how words go together to form sentences is especially important if we try to learn another language when we are older. To advance to learn another language, or to use our native tongue properly, in all circumstances, we go back to basics and learn the rules.
Most of us came to bird watching the same way. We started slowly at first, perhaps at a home bird feeder. Then we moved farther afield and added more species. But there were always a few birds that escaped our attempts to put a name on them. Perhaps it is those streaky sparrows that give us trouble, or female ducks, or immature gulls. Like language, in order to advance in birding, we need to go back and learn the basics. We don't need to memorize more field marks (build a bigger birding list or "vocabulary")--we need to learn how to look at birds and how they are put together.
Advanced Birding, 2011 by Kenn Kaufman.
Birding Essentials, 2007 by Jonathan Alderfer and Jon L. Dunn.
Birding Basics, 2002 by David Sibley.
The books above are quite similar, all excellent, and all serve the same general purpose... to teach us how to advance in our bird spotting and identification skills. But notice that "advanced" to these authors is synonymous with understanding the "essentials" and the "basics" of identifying birds.
For argument's sake, let's define an "advanced birder" as one who can quickly and accurately identify nearly every bird seen... near or distant, well-studied or barely glimpsed, or even heard-only. (There are, of course, some individual birds that even experts can't name after extensive study, but we're not talking about those right now.)
Identifying nearly every bird you can see is not about memorizing some secret and subtle field mark. First and foremost, it is about learning the basics of how to look at birds and "understanding what you see and hear," as is the subtitle of Kaufman's book.
Like a toddler learning the parts of the face, a birder needs to intimately understand the parts of a bird, including feather groupings and names. These are often called "topology" in the introduction of many field guides. As Kaufman says, "understanding the visible structure of the bird may do more than anything else to enhance your skill at identification."