Final answer:
The third and fourth stanzas create a sense of anticipation and the use of the word 'feather' emphasizes its significance in the poem.
Step-by-step explanation:
To begin with, the third and fourth stanzas make up one complete sentence, with a colon at the end of the third announcing the fourth; this helps achieve a sense of building up towards something important.
Then, we move from the visual image of a large spacious moor to the very circumscribed or restricted place where the feather is found, but the reason why this 'hand's-breadth' shines out is delayed for the next two lines.
'For there I picked up on the heather'-Okay, what did you pick up?—'And there I put inside my breast'-still don't know what you picked up—‘A moulted feather’—ah, I see! Notice the internal rhyme of ‘feather' with 'heather,' which draws attention to, and emphasizes the harmony of, the moment.
Also, the word ‘feather’ is repeated and expanded: ‘an eagle-feather.'
Clearly, the feather of no other bird would do, for, ultimately, the comparison is of the eagle to the poet; Browning knows Shelley through his poetry as he knows the eagle through its feather, and that feather presents a striking visual image.