from Thoreau's Journal, March 9, 1855:
You are always surprised
by the sight of the first spring bird or insect ;
they seem premature,
and there is no such evidence of spring as themselves,
so that they literally fetch the year about.
It is thus when I hear the first robin or bluebird,
or, looking along the brooks, see the first water-bugs out circling.
But you think,
They have come, and Nature cannot recede.