I've enjoyed the various discussions of the DOTT. As Gunnar pointed out, it's a topic I took on several years ago, with great gobs of verbiage but too little insight, before being dressed down by the much-missed Jack Kroll.
A more obvious and, I believe, more helpful reading than I'd previously suggested now occurs to me, inspired in part by some of the recent posts.
The thrush's song is part of a larger motif, like the laughter of the children in the apple tree, or the voice of the hidden waterfall -- the suggestion of bliss through sounds that suggest what lies beyond our view, but cannot be apprehended. The sounds are themselves part of a larger motif of intense revelatory but imcomplete sensory experience, like the winter lightening (sight), or the wild thyme unseen (smell), the bitter apple and the bite in the apple (taste) and the live nettle (touch) or the deeply-heard music (hearing again).
The thrush is a step closer to us than the other aural manifestations because, unlike the children or the waterfall, it briefly appears but, as birds are wont to do, disappears after giving just a snippet of its song. Because it can be seen only fleetingly, and cannot be restrained from disappearing, it provides a transition between the world we apprehend, and that we only intuit.
In this transitional capacity, the thrush leads us to the state where we experience the "hints and guesses" of what lies beyond, but does not lead us further. The suggestion that it may lead us to understanding is its "deception." (In this regard, the thrush sounds a lot like Eliot, or poets in general, though I have no basis to say he intended that association.)
I can't say that I've developed any sort of comprehensive analysis oalong these lines, or that I've nailed down even in my own mind the thrush-theme. But what I've set down above reasonably conveys what I have in mind. I wonder if others might have thoughts about Eliot's use of in FQ of the five senses to apprehend pieces of the puzzle -- perhaps examples beyond the ones I've noted, especially other examples of touch, which seems (perhaps deliberately) the least employed sense in suggesting apprehension of the next world?