I won't go into great detail here but will just mention my chief impressions. The first big improvement is the addition of many new species that were deemed too rare in North America to feature in the first editions. Consequently, there is a lot of useful new information that will appeal to serious birders. The treatments of regularly occurring species have also been enhanced, invariably for the better, with improved notes, new illustrations and updated maps. The quality of the little comments that David has added to the plates is very high, and it is a delight to see in print a lot of the new identification knowledge that's been kicked around over the past few years. In this regard the book is remarkably up to date.
I understand there have been issue with colors in various printings but my copies seemed fine with the glaring exception of the South Polar and Great Skua plates. The former are all too dark and gray while the latter are way too orange. In truth there is far more overlap and these extreme representations are potentially misleading. Some of the new paintings are lovely. I would relish a poster-sized version of the heron heads in their vibrant courtship colors. A few are so-so (Bulwer's Petrel and Black-bellied Storm-Petrel) but do the job.
The continued lack of scientific names for subspecies is a disappointment. I understood the original argument for keeping things simple so as not to put off those just getting into birding but now with the addition of extreme rarities and a greater sophistication overall, I think it's time to bite the bullet and include scientific names for recognizable subspecies. This will really help the user at every level connect to other information sources. For example, it's currently very difficult to match the expanded treatment of the Cackling and Canada Geese with literature and images that easily available on the web that more often use the scientific name. So for instance Sibley's "Pacific" Cackling Goose is not used elsewhere and it takes some digging to link this to minima, which is ubiquitous. Ambiguity is never good and I think there is ample room to include these important names. Likewise, it would be beneficial to add the group names of the various Fox Sparrows without necessarily listing every proposed taxa. My experience is that birders are keen to learn and one can lay out the information in such a way that it is accessible and useful to all levels. Maybe this could be touted as the major innovation in future edition?
This is already a fairly chunky book and it's almost too big to carry in the field. Many experienced birders believe this to be a good thing because it encourages advance study, although whether that sentiment is true in reality is another matter. Quite honestly, I would welcome an even larger format version (as was done with the Collins Guide) to see the illustrations better and perhaps have room for even more notes and come to think of it, scientific names for subspecies!
In summary, this remains very useful (arguably essential) item for anyone birding in North America. Without a doubt it's a substantial improvement on the already superb first edition and everyone should upgrade their copy. Of course the book is not the last word in bird identification and readers should use it alongside other guides, magazine articles, and of course the ever more abundant web resources.
Title: The Sibley Guide to Birds, Second Edition
Author: David Allen Sibley
Publication Date: March 2014
Format: Flexibound softback, 624 pages