As he became familiar with a pair of swallows in a nest-box close to his Maine cabin, biologist Bernd Heinrich wondered why they insist on white feathers—not easily available nearby—for their nest lining. Going deeper into his study of these remarkable birds, Heinrich notices, too, the extreme aggressiveness of "his" swallows toward other birds of their own kind. This meticulous but affectionate examination of the surprising, rich life of tree swallows in nesting season is rounded out by Heinrich's beautiful illustrations and photographs.