Try reading the poem out loud. How long does it take you? Probably less than 30 seconds, right? One of the things about this poem is that it reads very quickly. But if you slow down you'll find some interesting aspects of the sound.
The rhyme isn't completely regular, but there is rhyme here. There is only one end-rhyme (where the ends of lines rhyme directly), and that's with "Wheel" and "Cochineal." Other rhymes occur in the middle of lines, like "Evanescence" and "Resonance" or "Rush" and "Bush." In that way, the poem achieves a sense of symmetry, but does so in surprising ways. It's not like it's just slapping rhyme on the end of each line. By including internal rhyme, which pops up unpredictably in the middle of the lines, the poem neatly mirrors the way that this hummingbird has surprised the speaker with its beauty and ability.
We also get a good deal of alliteration (repeating beginning consonant sounds), with pairs of words like "Blossom" and "Bush" or "Resonance" and "Rush." Too, we get consonance (repeated consonant sounds in the middle or at the end of words). Consider the SH sounds of "Rush" and "Cochineal," or the soft S sounds of "Evanescence" and "Resonance." These repeated sounds give a kind of pleasant connectedness to the poem, one that the speaker might well feel for the hummingbird. The soft sounds, too, add to the easy, relaxed tone of the poem, making you feel like you're taking a walk on a slow and lazy summer day.