Thoreau writes in his journal:
It is a pleasant sound to me, the squeaking and the booming of nighthawks flying over high open fields in the woods. They fly like butterflies, not to avoid birds of prey but, apparently, to secure their own insect prey. There is a particular part of the railroad just below the shanty where they may be heard and seen in greatest numbers. But often you must look a long while before you can detect the mote in the sky from which the note proceeds.
