These books share the same illustrations and maps, and very similar text, differences are mainly in the size of the books and the inclusion of species and variations.
Summary: The big guide is more of a reference book and is recommended when you want to have access to all species and as much information as possible about every species. The smaller Eastern and Western Guides are true “Field Guides” and are recommended when you want a more portable size and a book that focuses on the species and variations you are most likely to see.
Size: The Eastern and Western guides are much smaller. At just under 5 by 8 inches, and under 1.5 pounds, they will fit into many pockets. The “Big Sibley” is larger at 6.25 by 9.75 inches, and nearly twice as heavy at 2.5 pounds. It will fit in a large jacket pocket, but generally has to be carried in a backpack or other bag.
Some benefits of the larger size of the big Bird Guide is that maps are larger (about 10%) and illustrations are larger (variable but up to 20% or more), and there is room for more illustrations and text.
Species included: The big guide covers over 900 species – all of the regularly occurring species (and some rarities) found in North America north of Mexico. The smaller Field Guides leave out the species that are absent or very rare in that region. The Western Guide includes about 715 species and the Eastern guide includes about 650 species.
Variations included: To make room for all of those species in a smaller format, the Eastern and Western guides include fewer images of each species. Species that are rare in a region have very limited coverage with only one to three illustrations, and even many common species have the number of illustrations reduced, with some subtle or seldom-seen variations dropped.
Differences in content: The text for many species is changed slightly between the three books. Mostly this reflects regional differences in status or other features, but in some cases text was also rewritten to reflect different identification challenges in each region. Some text simply wouldn’t fit in the smaller books and was cut (for example, voice descriptions are shortened), and the sidebar essays on various topics are slightly different among the three books. The revised Eastern and Western field guides were published two years later than the big guide, and include several species of introduced exotic birds that are not included in the big guide. These are primarily found in southern California and Florida: Pin-tailed Whydah, Bronze Mannikin, Orange-cheeked Waxbill, and Cinnamon-rumped Seedeater, and Japanese White-eye.