What poetic element, like rhyme or rhythm, is supported in the interpretation of "A Bird came down the Walk"?

Poetry has lyrical origins, as people in ancient times tried to preserve and pass down their culture and history. As such, poetry is imaginative and uses words (diction) and elements of poetry, such as meter, rhythm, rhyme, tone and figurative language to infer meaning in such a way that it is able to express intention and attach significance which may otherwise be overlooked or misunderstood outside a certain context. It may even hold secrets and special meaning which, without interpretation, go unnoticed.

In the simply-worded "A Bird came down the Walk—," Emily Dickinson uses various poetic elements to make her point. She demonstrates that Nature reveals itself in absolute simplicity and there is no need to be a great scholar with a sophisticated vocabulary to appreciate Nature. 

The first two stanzas follow a familiar pattern, the second line rhyming with the fourth. However, although the metrical rhythm and flow is sustained (the punctuation confirms this), there is no rhyme in the subsequent verses; notably, to ensure that the reader is as equally affected by the bird's actions as the observing narrator. Dickinson uses familiar sound devices (particularly alliteration) to maintain the rhythm and ensure the progression of the poem. The words are familiar but the way Dickinson uses them is what gives this poem its power, its effect. 

Dickinson understands the ruthlessness of Nature; the bird "ate the fellow raw." Personifying both the bird and the worm ensures that the reader sees more than a basic act between a bird and worm, a natural and seemingly insignificant event, and relates this circumstance to his or her own life, especially as the bird "hopped sidewise to the Wall," such as a person might dismiss something they have no particular interest in in the pursuit of their goals.

When the bird and the narrator encounter each other, the tone changes as both the bird and narrator become "Cautious" and alert. The narrator's attempts to placate the bird, despite her own fear, fail because the bird uses its most natural instinct and flies away. The observing narrator notes how seamless the action is. Interestingly, Dickinson would rather compare the beauty of flight and the bird's wings to "Oars (that) divide the Ocean," because to her there is splendor in flight that can only be understood and magnified by considering the vastness of the ocean. She then uses the image of a butterfly to cement the visual picture she has created. There is no need to mention color when describing a butterfly; the narrator expects that it will be understood. This use of imagery is wholly supported as the entire poem remains with the reader. The harsh reality is far outweighed by the breathtaking and overwhelming vision with which Dickinson leaves the reader.