For the youngest, a fine concept book. Always as much naturalist as artist, Arnosky begins with a very few basic, concisely stated (but not simplistic) ideas, pointing out that there are ``shorebirds, land birds, and water birds,'' that some can't fly, that ostriches are biggest and hummingbirds smallest, and even describing the wing motions for flying forward, hovering, or gliding. His handsome portraits—memorably clear and vivid—graphically emphasize the birds' most significant features. The ordering is informal but logical (swan, goose, and duck; stork and heron; chicken and turkey seen together). This is not a book that differentiates mallard from Muscovy, adult from juvenile, or even male from female; it's simply an introduction- -and an excellent one. Also available: Crinkleroot's 25 Fish... (ISBN 0-02-705844-1); (Nonfiction/Picture book. 2-8)