in Thoreau’s Journal:
Not finding the birches, I returned to the first swamp and tapped two more white birches. They flow generally faster than the red or white maples when I tried them. I sit on a rock in the warm, sunny swamp, where the ground is bare, and wait for my vessels to be filled. It is perfectly warm and perhaps drier than ever here. The great butterflies, black with buff-edged wings, are fluttering about, and flies are buzzing over rock. The spathes of the skunk-cabbage stand thickly amid the dead leaves, the only obvious sign of vegetable life. A few rods off I hear some sparrows busily scratching the floor of the swamp, uttering a faint tseep tseep and from time to time a sweet strain. It is probably the fox-colored sparrow. These always feed thus, I think, in woody swamps, a flock of them rapidly advancing, flying before one another, through the swamp. A robin peeping at a distance is mistaken for a hyla. A gun fired at a muskrat on the other side of the island towards the village sounds like planks thrown down from a scaffold, borne over the water. Meanwhile I hear the sap dropping into my pail. The birch sap flows thus copiously before there is any other sign of life in the tree, the buds not visibly swollen. Yet the aspen, though in bloom, shows no sap when I cut it, nor does the alder. Will their sap flow later? Probably this birch sap, like the maple, flows little if any at night. It is remarkable that this dead-looking trunk should observe such seasons, —-that a stock should distinguish between day and night.