The United States is blessed with birds, a vast and richly varied population that changes considerably as the habitat changes from forest, to meadow, to marsh. As Thomas P. McElroy puts it in this unique birding guide, "Birds are everywhere . . . and this fact alone provides innumerable ecological equations to challenge the most curious of intellects." This is not a guide that answers only "What bird is it? but the deeper, more fascinating questions such as these: Why is it there? How does it survive? How does it contribute to the ecology of its surroundings? The answers to these questions, and many more, carry the birdwatcher well beyond lists and into a more involving, more satisfying means of observing nature.