Thoreau writes in his journal:
Such a habit have cows in a pasture of moving forward while feeding that, in surveying on the Great Fields to-day, I was interrupted by a herd of a dozen cows, which successively passed before my line of vision, feeding forward, and I had to watch my opportunity to look between them. Sometimes, however, they were of use, when they passed behind a birch stake and made a favorable background against which to see it.