in Thoreau’s Journal:

Some grass is seen to wave in the distance on the side of N. Barrett’s warm hill, showing the lighter under sides. That is a soft, soothing, June-like impression when the most forward grass is seen to wave and the sorrel looks reddish. The year has the down of youth on its cheek. This, too, is the era of the bobolink, now, when apple trees are ready to burst into bloom. Now it is too late to retreat from the summer adventure. You have passed the Rubicon, and will spend your summer here. Lately, for a few days, the note of the pine warbler rang through the woods, but now it is lost in the notes of other birds. Then each song was solo. Its vetter vetter vetter vetter rang through silent woods. Now I rarely hear it. A yellow butterfly.