Watching and identifying birds quickly leads to curiosity about how and why birds do the things they do. How does a tiny chickadee survive subzero temperatures? How does an albatross cross miles of ocean without flapping? Why do male wood-warblers have two distinct song types? The answers, or proposed answers, exist in technical papers and in textbooks, but such resources are often inaccessible to birders, and the information of most interest to amateurs is frequently embedded in a wealth of technical detail. Our aim in this book is to provide an introduction to the great variety and complexity of bird life -- a book written by and for birders that will help readers interpret and understand the things they see in the field. - Introduction.