Vainly the fowler's eye Might mark thy distant flight to do thee wrong, As, darkly painted on the crimson sky, Thy figure floats along.
"Vainly"—that's the word that begins the poem's second stanza. So, what does it describe?
It's a bit tricky, but if you read through the stanza, "vainly" describes the fowler's (a guy who hunts birds) attempt to "mark," or follow, the waterfowl's journey across the sky.
In other words, it would be in vain (useless) for the fowler to follow the path in an effort to kill it. "Do thee wrong" is without question a euphemism for "hunt" or kill. (Yeah, if somebody blasted us out of the sky, we'd think he'd done us wrong, and then some.)
It would be impossible for the fowler to track the bird with his eye because the bird's figure, or outline, or silhouette, is "darkly painted on the crimson sky," floating along.
The sky is crimson, and the bird looks dark against that backdrop, which means it almost blends in. It's almost like the bird is camouflaged.
Recall that, in the first stanza, the sky was already "rosy." Now it is crimson.
The speaker could just be getting creative, or changing up his colors, or he could be suggesting that the night is coming on fast and that the sky is now darker than it was before.
Now there's talk of hunting and a very dark—crimson—sky. We thought this might be a depressing poem, and things are for sure heading in a very unpleasant direction, to say the least.
Before we go on, though, let's say something about the waterfowl's movement. It "floats along." It doesn't flap its wings; it doesn't race through the sky. It simply "floats."
In a way, it almost seems dead—just floating in the air, born up by the wind. At the same time, there's something very graceful about its movement through the air. It is elegantly, quietly, "floating."
We wonder if this very delicate description of the bird will hold as we continue to read the poem, so let's keep it in mind as we move forward to the poem's third stanza...