in Thoreau’s Journal:  

In this clear air and bright sunlight, the ice covered trees have a new beauty, especially the birches along under the edge of Warren’s wood….bent quite to the ground in every kind of curve….The birch is remarkable, perhaps, from the feathery form of the tree, whose numerous small branches sustain so great a weight, bending it to the ground; and, moreover, because, from the color of the bark, the core is less observable….The birches droop over in all directions, like ostrich feathers.