There are 3 worthy North American field guides. But the one I carry with me on trips, the one I turn to first, has been updated to compete strongly with the others. Yes, with its 2011 printing, Jon L. Dunn and Jonathan Alderfer did it again with the National Geographic Field Guide to the Birds of North America, now in its 6th Edition!
The back cover advertises: "America's #1 Bird Guide Just Got Even Better!". While I don't necessarily agree with the grammar, I agree with the thought.
You may argue that The Sibley Guide to Birds is your favorite. Fine. That was an amazing book when it first came out, and is still a strong contender. The songbirds shown in flight in that book still haven't been matched by any other guide. But when Sibley's guide first hit the market in 2000, the National Geo was in its 3rd Edition with just over 800 species shown, matching the 810 in Sibley. The 5th Edition of the National Geo was a complete make-over, and this 6th Edition is also a redesign--now with 990 species (including 92 Accidentals and Extinct)!
While many of the illustrations in the National Geo are familiar through all versions of the book, this 6th edition claims 300 new art pieces in addition to all the changes in the 5th Edition! Averaging over 3 illustrations per species (as opposed to 8 for Sibley), the National Geo's bird illustrations are larger and more detailed than Sibley's. I noticed many new illustrations, including standing and close-up head views of jaegers. I notice the goatsuckers no longer have "shrunken heads" as those illustrations were re-done. Many of the warblers were re-drawn. The comparison views of the foreheads of Tundra and Trumpeter Swans are a great new ID illustration.
This new edition places helpful identification text next to the illustrations, making it similar to the arrows and text in Sibley. This seems to add almost 50% more identification text than the previous edition of this guide. Imagine having an expert write additional ID comments next to each illustration in your field guide. Wow!
Following the lead of The Stokes Field Guide to the Birds of North America the new National Geographic guide also heavily stresses field identifiable subspecies, with 59 maps showing subspecies in the main text, and an additional 37 subspecies maps in the appendix.
The subspecies are especially helpful for the white-cheeked geese, carefully delineating range and plumages of the various populations of Canada Goose and Cackling Goose--something birders really need, based on the amount of confusion I have witnessed among birders.
The maps are updated, too. The 3-color maps of the 5th Edition (breeding, winter, year-round) have been replaced with 6-colors (adding 3 different colors for migration: spring, fall, both). Hurray!
The new edition also adds more voice annotations. For instance, previous versions of the guide did not list the distinctive calls of swallows and some shorebirds (Red Knot, Surfbird, Rock Sandpiper)--it does now.
Finally! We now have a field guide that shows North American seabirds correctly and completely! It is updated with rarities and subspecies that may actually be separate species. The illustrations of wing molt in Wilson's Storm-Petrel was a nice surprise.
A new feature is a quick-find index on the front cover, and a visual index to bird families on both front and back covers. These will help newer birders find birds and learn the taxonomic sequence.
All the way around, this is a great field guide. In fact, compared to previous editions (especially the 4rd Edition or earlier), this seems like a brand new field guide to hit the market!
Related: A review of the new Stokes guide.