Joe, I just remembered another book which might best fit your need for a portable supplement to the National Geo guide. _Advanced Birding_ by Kenn Kau(f)fman goes into great detail about several of the more difficult ID's to make. I don't have the book in front of me, so I can't tell you exactly what's in it, but I know there are chapters about gulls, flycatchers, accipiters, sparrows among several others. The book is about the size of the Peterson field guide, but lighter in weight. If you want more specific info, email me. If you haven't already, check it out at a bookstore. The information in it is, from all accounts I've heard, very accurate and comprehensive. It might be just the kind of book you're looking for. Andy Rabin Gaithersburg, MD andyrab@wam.umd.edu