Even though it's not identified outright, the hummingbird gets described in multiple ways throughout this short poem. Think of the poem almost like a Cubist Picasso painting—rather than giving us a unified image of a bird, the speaker chops it up and represents it as discrete parts (its wings, its color, etc.). As the star of the show, though, the hummingbird itself becomes a symbol of the otherworldly possibilities of the world around us.
Line 1: "A Route of Evanescence" is a paradox—how can something be a "path of disappearance"? Don't routes actually signal the presence of something? There's immediately a tension between presence and absence, and the hummingbird seems to be the crux of this. The speaker anticipates how, looking at the blurred motion of the hummingbird, her mind will soon be off on an abstract, invisible route.
Line 2: In comparing the hummingbird's wings to a "Revolving Wheel," the speaker creates a complicated metaphor. It seems to suggest a sense of constant motion but also awe, the kind of awe you might experience when seeing something like a train for the first time.
Line 7: If you were completely confused when you read the line, "The mail from Tunis, probably" never fear! Dickinson was a big Shakespeare fan, and this line is actually a reference to Shakespeare's play The Tempest, about an exiled magician trying to seek revenge on his enemies. In Act II, Scene 1 of the play, Antonio and Sebastian are newly stranded on an island in the middle nowhere. Convinced that their King Alonso is dead, Antonio begins making plans to take over the throne. He claims that, since the King's daughter just got married to Tunisian royalty, she'd rather stay in Africa than come back to Naples to claim the throne. As the two men plot, Antonio describes how far away she is: "She that is Queen of Tunis; she that dwells / Ten leagues beyond man's life; she that from Naples / Can have no note, unless the sun were post." The idea here is that the city of Tunis seems so many galaxies away that the only way one could deliver or receive news is if the sun delivered the mail! Dickinson uses this allusion to give the hummingbird an out-of-this-world quality. This creature that we take for granted as a simple domestic hummingbird? With a single reference to Shakespeare, Dickinson turns the bird into a magical being from far, far away.