The Sibley Guide to Birds provides a wealth of new information:

• Captioned illustrations show many previously unpublished field marks
  and revisions of known marks
• Nearly every species is shown in flight
• Measurements include length, wingspan, and weight for every species
• Subspecies and geographic varients are covered thoroughly
• Complete voice descriptions are included for every species
• Maps show the complete distribution of every species: summer
  and winter ranges, migration routes, and rare occurrences

Both novice and experienced birders will appreciate these and other innovative features:

• An introductory page for each family or group of related families
  makes comparisons simple
• Clear and concise labels with pointers identify field marks directly
• Birds are illustrated in similar poses to make comparisons between
  species quick and easy
• Illustrations emphasize the way birds look in the field