The joyful tone of "To a Butterfly" is further developed with this line:

We'll talk of sunshine and of song

The speaker here is addressing his comments to a motionless butterfly which he has been observing for "a full half-hour." He begs the butterfly to take rest near him as if their environment provides a "sanctuary" from the labors of flying. And while he rests, the butterfly and the speaker can talk about "sunshine" and "song."

These two bright metaphors convey a light ease in the enjoyment of nature. Sunshine often symbolizes joy, contentment, and enlightenment. Song often symbolizes creativity, an overflow of emotion, and felicity.

Therefore, the butterfly brings out positive emotions in the speaker, and he feels compelled to share his joy with the world around him—even the butterfly. The butterfly also reminds him of "summer days" he spent with his sister years ago, when time seemed to pass luxuriously slowly. Thus, perhaps he also seeks to bring his sister into this conversation of "sunshine" and "song" and recall days with a more simple focus, such as the innate fascination of watching a butterfly.