Just so simple is every year.

April 5. 

These days, when a soft west or southwest wind blows and it is truly warm, and an outside coat is oppressive, — these bring out the butterflies and the frogs, and the marsh hawks which prey on the last. Just so simple is every year. 

Whatever year it may be, I am surveying, perhaps, in the woods; I have taken off my outside coat, perhaps for the first time, and hung it on a tree; the zephyr is positively agreeable on my cheek; I am thinking what an elysian day it is, and how I seem always to be keeping the flocks of Admetus such days — that is my luck; when I hear a single, short, well- known stertorous croak from some pool half filled with dry leaves. 

You may see anything now — the buff-edged butterfly and many hawks — along the meadow; and hark! while I write down this field note, the shrill peep of the hylodes is borne to me from afar through the woods.

H. D. Thoreau, Journal, April 5, 1854

These days bring out the butterflies and the frogs, and the marsh hawks which prey on the last. Just so simple is every year. See April 8, 1854 ("Methinks I do not see such great and lively flocks of hyemalis and tree sparrows . . .Perchance after the warmer days, which bring out the frogs and butterflies, the alders and maples, the greater part of them leave for the north and give place to newcomers.")

I have taken off my outside coat, perhaps for the first time, and hung it on a tree when I hear a single, short, well- known stertorous croak from some pool half filled with dry leaves. See April 13, 1853 (" First hear toads (and take off coat), a loud, ringing sound filling the air, which yet few notice.")

Field note. See March 21, 1853 ("Might not my Journal be called 'Field Notes?'")

The shrill peep of the hylodes is borne to me from afar through the woods. See April 5, 1858 ("I hear the hylodes peeping now at evening, being at home, though I have not chanced to hear any during the day."). See also  March 23, 1859 ("We hear the peep of one hylodes somewhere in this sheltered recess in the woods."); March 31, 1857  ("The shrill peeping of the hylodes locates itself nowhere in particular.")  April 1, 1860 (" I hear the first hylodes by chance, but no doubt they have been heard some time.");  April 2, 1852 ("I hear a solitary hyla for the first time.");  April 3, 1853 ("At Hayden's I hear hylas on two keys or notes. Heard one after the other, it might be mistaken for the varied note of one.");  April 6, 1858 ("I hear hylas in full blast 2.30 P. M.")