To understand the last stanza of Dickinson's poem, you really need to read the last two stanzas together and understand the structure of the poem as a whole. The first three stanzas describe the natural actions of the bird before he realizes he is being observed - he hops along the walk, eats a worms, drinks some dew, hops to the wall, then nervously looks about.
It is then, feeling "like one in danger," he turns his "velvet head" and sees that he is being observed (Dickinson 12-13). The speaker, "cautious," offers the bird a crumb, but the bird quickly "unroll[s] his feathers" and takes to flight (15).
The last stanza describes the manner in which the bird flies away, using imagery to show just how gracefully the bird moves through the air. The fifth (and last) stanza really begins with the last two lines of the fourth stanza:
... he unrolled his feathers
And rowed him softer home
Than oars divide the ocean,
Too silver for a seam,
Or butterflies, off banks of noon,
Leap, plashless, as they swim.
Dickinson uses both sea and land imagery here, saying the bird's wings "[row] him softer home/than oars divide the ocean," appearing "seam[less] in the sky (18); the bird is even more graceful than butterflies leaping off the bank, "plashless" (without a splash) as they "swim" through the air (19-20).
The speaker watches, amazed, at the beauty, grace, and freedom of the bird as it flies away.