March 17. P. M.—To Walden and Goose Pond. [...] I see a large flock of sheldrakes, which have probably risen from the pond, go over my head in the woods. A dozen large and compact birds flying with great force and rapidity, spying out the land, eyeing every traveller, fast and far they "steam it" on clipping wings, over field and forest, meadow and flood; now here, and you hear the whistling of their wings, and in a moment they are lost in the horizon. Like swift propellers of the air. Whichever way they are headed, that way their wings propel them. What health and vigor they suggest! The life of man seems slow and puny in comparison,—reptilian.
April 4. [...] The birds sing quite numerously at sunrise about the villages,—robins, tree sparrows, and methinks I heard the purple finch. The birds are eager to sing, as the flowers to bloom, after raw weather has held them in check.
April 25. [...] I hear the greatest concerts of blackbirds,—redwings and crow blackbirds nowadays, especially of the former (also the 22d and 29th). The maples and willows along the river, and the button-bushes, are all alive with them. They look like a black fruit on the trees, distributed over the top at pretty equal distances. It is worth while to see how slyly they hide at the base of the thick and shaggy button-bushes at this stage of the water. They will suddenly cease their strains and flit away and secrete themselves low amid these bushes till you are past; or you scare up an unexpectedly large flock from such a place, where you had seen none.
May 16. P. M.—To Copan and Beck Stow's. [...] Think how thoroughly the trees are thus explored by various birds. You can hardly sit near one for five minutes now, but either a woodpecker or creeper comes and examines its bark rapidly, or a warbler—a summer yellowbird, for example—makes a pretty thorough exploration about all its expanding leafets, even to the topmost twig. The whole North American forest is being thus explored for insect food now by several hundred (?) species of birds. Each is visited by many kinds, and thus the equilibrium of the insect and vegetable kingdom is preserved.
H. D. Thoreau