Sparrows are as complicated as they are common. This is an essential guide to identifying 76 kinds, along with a fascinating history of human interactions with them.   What, exactly, is a sparrow? All birders (and many non-birders) have essentially the same mental image of a pelican, a duck, or a flamingo, and a guide dedicated to waxwings or kingfishers would need nothing more than a sketch and a single sentence to satisfactorily identify its subject. Sparrows are harder to pin down. This book covers one family (Passerellidae), which includes towhees and juncos, and 76 members of the sparrow clan.     Birds have a human history, too, beginning with their significance to native cultures and continuing through their discovery by science, their taxonomic fortunes and misfortunes, and their prospects for survival in a world with ever less space for wild creatures. This book includes not just facts and measurements, but stories--of how birds got their names and how they were discovered--of their entanglement with human history.