A repost from 2005:
Birding (it used to known as bird-watching but that didn't sound cool, and birders are of course cool) is the most popular and rapidly growing form of outdoor recreation in America.
Besides all of the excellent birding handbooks that are out these days, the Cornell Laboratory of Ornithology has a good basic site for basic info and identification, All About Birds, which we use all the time. They also have an advanced site, The Birds of North America, which is by subscription.
Other things that CLO offers: A home-study course on bird biology, behavior, and identification and the necessary A Field Guide to Bird Songs.
Photo: Two warblers after banding. These are immature birds in fall plumage, a Mourning Warbler on top and a Connecticut Warbler below.