Emily Dickinson's poem focuses on how humans frequently want to ascribe human motivation to animal behavior while downplaying the animal's instinctive understanding of its surroundings. In doing so, she is able to illustrate both the beauty and brutality of nature.
In the first stanza, the speaker seems to suggest that perhaps the bird would not have been so brutal ("ate the fellow raw") if it had known it was being observed ("He did not know I saw"). By making this observation, the speaker might be suggesting that the bird's instincts are somehow inferior to human interactions.
In the second stanza, "And then hopped sidewise to the Wall/To let a Beetle pass" suggests that the bird makes a conscious decision to share space with the beetle, much like a person might step aside to avoid a confrontation or a traffic jam on a busy city sidewalk. This anthropomorphism possibly implies that the human response is superior to the more instinctive motivation natural to the bird.
The third, fourth, and fifth stanzas emphasize the dangers as well as the beauty of nature. These stanzas are the least tied to the speaker's earlier anthropomorphic perspective. It is true that the description of the eyes of the bird as "beads" and its head as "velvet" does take the emphasis off nature since both of those materials would be man-made, but there does not seem to be a suggestion of the superiority of humans in either of those comparisons.
In stanza five, the speaker describes the movements of butterflies in terms of swimming. It might be fair to say that the speaker's inability to describe the bird's or butterflies' physical attributes without comparing to common images hints that human language is sometimes more limiting than instinct is in the natural world.