I agree with what lit24 and want to add a few more items addressing the second part of your question, on the themes in Hardy's poem "The Darkling Thrush." I write "themes," not "theme," because there's probably more than one. The enotes study guide identifies at least three:
1. Search for Meaning
3. Chaos and order
The first one is already hinted at in lit24's final paragraph. The speaker in the poem hears the birdsong coming from across the wintery wasteland. In the final stanza, the speaker says that he sees "So little cause for carolings" (i.e. few good reasons to burst out into song, or celebrate) and concludes that the bird maybe knows something that he doesn't:
Some blessed Hope, whereof he knew
And I was unaware.