I remember when I first encountered this poem in my first-year english class. It intrigued me, but I couldn't really make anything of it. Eventually I had to write an essay on it and I realized why it had interested me.
Now, before I continue, I want to clarify something. The word "punk" is very poorly defined and, due to its incorporation as the latest brand-name, encompasses a lot of different things. What I speak of here is what the core of punk is to me.
I think The Darkling Thrush is about punk philosophy. The idea of punk springs from the conception that the world is hopelessly corrupt, tarnished, dirty, and irredemably meaningless. This is a fairly obvious parallel to the first two stanzas of the poem.
Punk then, is an acceptance of this wasted world and a "so be it" attitude. The world may be lost, but that's no reason not to live your life in a way that is meaningful to you. The Thrushs enthusiasm and courage represent the creation of beauty for its own sake, despite its actual meaninglessness.