And she balanced in the delight of her thought, A wren, happy, tail into the wind, (4-5)
With words like "delight" and "happy," and with the image of the wren, these lines certainly have a positive, cheerful vibe. The word "balanced" also stands out. When we think of balancing, it usually brings to mind things that are a little difficult: a balance beam, balancing many responsibilities, balancing your finances. Perhaps it is a little tricky for Jane to maintain the sense of delight and happiness that these lines discuss.
Her song trembling the twigs and small branches. (6)
The wrens song causes some "trembling." Remembering that the wren is the metaphorical equivalent of happy-Jane, it is Jane's happiness that is causing this trembling. Trembling is a very delicate, fragile kind of movement. It is certainly possible to tremble with happiness (think back to Christmas morning when you were 5 or 6), but it is an action more associated with fear or sadness (piano recital, scary moving, sad movie, take your pick).
The shade sang with her; […] And the mold sang in the bleached valleys under the rose. (7-9)
Here's where we can see that contagious nature of happiness at work. Shade is nice in the summer time. A nice shady spot is a great place for a nap (as opposed to napping with your head on the keyboard when you should be trying to finish your paper on "Elegy for Jane." Plus, drooling on your keyboard can wreck your laptop. Believe us. We found out the hard way). While shade is great in the summertime, this poem doesn't have a very summer-y vibe. That "damp grave" setting feels a little more winter-y. Shade is often associated with darkness or the cold—the opposite of warm sunlight. So, we have shade and we have mold (representing decay and, perhaps, even death) and yet both of these negatives are moved to join in when they hear the wren's happy song. Happiness might be fleeting, but it's some pretty powerful stuff.