I thought I'd re-post Mark Halliday's useful comment just added on another thread, as his concern with how we know, and how we might both assert what birds know and doubt that assertion at the same time, seems relevant here.

"Hardy constructs that last sentence of "The Darkling Thrush" to balance disbelief against faith so delicately and evenly that you're never sure whether despair or hope emerges as the dominant mood. I think of it as one of the great poetic epitomes of agnosticism. The thrush does seem to know something, and Hardy doesn't dismiss such knowing as ridiculous, and yet he remains unaware of the hypothetical blessed Hope. Though he positioned himself as a rationalist in response to pious believers, Hardy loved to create these moments of tenable agnosticism; another great example is the end of "The Shadow on the Stone" where he chooses not to destroy his illusion that his wife's ghost is with him in their garden after her death."

We know that birds and animals possess knowledge. Undeniably, the thrush knows how to find food, construct a nest, make songs; what else does it know, and how? To assert too firmly that we can read and interpret their awareness feels false, but likewise to say that all attempts to understand animal consciousness are just vain projections seems too easy. Keats convinces in his ventriloquism because it's so headlong and confident,and the music is so gorgeous that there's a thush-ness rising from the page. Bunting doesn't want to convince us that his is the monologue of a thrush, but rather a witty and artfully dour commentary on human capacity-- specifically the poet's capacity -- for sentimentality and misreading. Hardy does seem to stand, as Mark H notes, in this double ground that feels distinctly modern -- I know the bird sings, I know what the song might mean, and I am also incapable of reading that song's intention, and understand what I'm lending to it as a listener.

I used to know a fellow who studied SIddha yoga, which involves practicing various meditation techniques that are intended to lead one to various capabilities of higher consciousness -- superpowers, basically. One of them is the ability to understand the speech of animals. He said he overheard the conversation of two dogs walking down a road, and that it was really boring, something along the lines of I want to eat that, smells good, yep. I don't know if I believe this story, but it reminds me that there's something about assuming the birds in question here have come to address the poet that's a little on the anthrocentric side. It's a pleasure, in Hardy, that the bird is striking his note of hope without any apparent regard for audience.