Analysis of About The Nightingale

From a letter from STC to Wordsworth after writing The Nightingale:

In stale blank verse a subject stale
     I send per post my Nightingale;
     And like an honest bard, dear Wordsworth,
     You'll tell me what you think, my Bird's worth.
     My own opinion's briefly this--
     His bill he opens not amiss;
     And when he has sung a stave or so,
     His breast, & some small space below,
     So throbs & swells, that you might swear
     No vulgar music's working there.
     So far, so good; but then, 'od rot him!
     There's something falls off at his bottom.
     Yet, sure, no wonder it should breed,
     That my Bird's Tail's a tail indeed
     And makes it's own inglorious harmony
     Æolio crepitû, non carmine.