Lines 17-20

At once a voice arose among
The bleak twigs overhead
In a full-hearted evensong
Of joy illimited;

• So the first thing that you've probably noticed is that we're taking on a whole four lines in this section. We're moving up, folks.
• Don't worry, we're not trying to get out of explaining the poem line-by-line. It's just that the third stanza marks a significant shift in the poem. See, up until now, the poem seemed to break itself up into four-line chunks. How do we know this? Well, for one thing, there's a period every four lines or so.
• Now, though, things are changing. At the end of line 20, we don't have a period. We have a semi-colon! Sure, it's half a period. But it's also half a comma. The sentence goes on for another – wait for it – four whole lines! It's like the poem is breathing a sigh of relief and opening up. Heck, who wouldn't want to talk about joy for a little bit longer?
• Why? Well, that's because things are looking up. All of a sudden, out of all that silence and death and never-ending grayness, our speaker hears something. And not just any sound – this is an all-out love song. It's full and beautiful and chock-full of happiness.
• But who's singing this happy song? We'll see….

Lines 21-22

An aged thrush, frail, gaunt and small,
In blast-beruffled plume,

• Now we're talking. After all, we were told in the title that this poem was about a thrush, right? It's taken us three and a half stanzas to get to the bird, though, and we bet plenty of you were beginning to wonder if the title was a weird attempt to fool us. But, three-quarters of the way through the poem, the thrush makes its star appearance.
• But wait. What kind of star is this? Old, weak, and tiny? This sounds like more of the gloominess that dogged us all the way through the first parts of the poem. If any of you were thinking that this poem might be at all like that other famous poem about a bird, you're probably re-thinking that just about now. After all, Keats's "Ode to a Nightingale" is full of pretty things and, well, a pretty bird.
• Whatever this bird is, though, it sure ain't pretty. Keats's poem is about the undying voice of the nightingale. The bird's immortal, for crying out loud. Hardy's bird, in comparison, is stuck in the middle of a nasty storm. The best thing that the speaker can say about the bird is that it somehow manages to exist in all of that feather-ruffling wind.

Lines 23-24

Had chosen thus to fling his soul
Upon the growing gloom.

• Jealous, are we? As frail and puny as this bird is, it's managed to do what our speaker has been too scared to do: to forget about the odds and just sing. Sure, the chances are that the bird won't be able to do anything to make the "growing gloom" one ounce lighter. But it's willing to try.
• Then again, it's managed to draw the attention of our speaker. And that's no small thing.