Having used all the bird guides for NA (iBird, National Geographic, Peterson and Audubon) the Sibley guide is simply the best for identifying birds. His illustrations contain a wealth of information; every little detail, whether or not it has caption, relates to an actual identifying characteristic of the bird in question. Sibleys illustrations are the best at portraying the general shape, posture and expression of the bird, aspects that are often more important then color in identifying a species. Not only are the illustrations superb, this guide has more of them per species then any other guide, showing different ages, plumages and differences between the sexes. It is also the only guide to illustrate every species in flight, a feature that is very useful. Another feature that is unique is the ability to compare two species side by side which helps immensely in making a final identification. Sibleys may not have all the rare and accidental species that NG has, and its search feature is not as advanced as iBird, but it still is the best at helping you make a positive identification.