A Route of Evanescence With A Revolving Wheel (1-2)
The poem begins almost with a paradox, as we start with a flight path that is evanescent, or fading. This helps to establish one of the major tensions in the poem between presence and absence, or between what the speaker saw and what she feels. In comparing the hummingbird's wings to a revolving wheel, the speaker suggests that a small bird can create just as much excitement as that great innovation, the wheel. A wheel that revolves may call to mind a large Ferris wheel, or even the wheels of a train, which was still considered a marvel in Dickinson's day. (We have to admit, we still kind of dig it.)
A Resonance of Emerald—A Rush of Cochineal (3-4)
Here the speaker describes the bird's colors using extremely specific language from the natural world. This tells us that the speaker is quite educated, because she knows color variations that occur in nature. The color emerald is named after a green gemstone. Similarly, cochineal is an insect found in South America and Mexico, ground up and used to make a crimson-colored dye that is found in lipstick and blush. Ew.
And every Blossom on the Bush Adjusts its tumbled Head (5-6)
At this point the speaker moves away from the bird and to the flowers that the bird has disrupted, either by crashing into them or zipping by them. The speaker personifies the flowers as they seem to wake up here, suggesting that the magical influence of the hummingbird has given them life. (That's just like us with coffee.)