Much has changed in the world of birding since the original edition was published in 2000. Many species have been documented for the first time in North America, and some birds considered rare have been shown to visit regularly. Other formerly common species are sliding toward oblivion. Some introduced species have established self-sustaining populations. Advances in genetic research have changed our understanding of evolutionary relationships among birds. As a result, many species have been split into two or more species, some genera (like Dendroica) have been eliminated, other genera (like Leucophaeus and Hydrocoloeus) have been created or restored, and families or even orders (like Falconidae and Anseriformes) have been moved to new positions on the AOU Checklist.
Technology, too, changed birding considerably. News of rare birds is now disseminated quickly over email lists, social media, and eBird alerts so that more birders can see rare birds when the opportunity arises. Anyone can use eBird to find good spots for birding. Digital cameras and sound recorders have made it easier to document rare birds and interesting bird behaviors; at the same time detailed written field notes have become less common. Birders also have easier access to more information (and unfortunately some misinformation) about bird identification and behavior.
The new edition reflects these changes. Despite being about the same size as the original, it includes 111 species that were omitted from the first edition. Many of these were first documented in North America in the years since the first edition was published. Others are the result of species splits or rare vagrants becoming more regular. The taxonomic order is up-to-date as of the 2013 supplement to the AOU checklist. Subspecies and regional populations also get more thorough coverage in the second edition, perhaps in anticipation of potential future splits.
Beyond those updates, many plates have been retouched or repainted entirely. Images have been enlarged, and the species accounts now contain more textual information about bird identification and habitats. While the first edition contained a number of sidebars, Sibley made room in the second edition for even more of them. For example, the sidebar on owling (shown below) appears in the second edition but not in the first.
Features that I liked best about the original Sibley Guide are maintained in the second edition. For one, I think David Sibley does the best job of any field guide artist (at least among North American field guides) of painting bird shapes that are true to life. Occasionally with other painted guides I see a bird shapes or postures that somehow do not look right. I have never experienced that with Sibley's illustrations. Shape is at least as important as plumage for accurate bird identification. In both editions, the diagrams showing bird topography in the introduction are tremendously useful. I refer to them constantly when I write up descriptions of rare birds to make sure my terminology is correct. The new guide also continues to show similar species in the same postures, and all but the most uncommon bird species are shown in flight.
A frequent criticism of the original Sibley Guide was that some of the colors, particularly orange, were too bright. It did not bother me very much, but it seemed like a fair criticism. That seems to be less of an issue with the new edition, which has more muted reds and oranges. Some reviewers have objected that certain colors are too dark. I agree with other reviewers that at least one bird, Scarlet Tanager, looks darker than it appears in the field (at least to my eye). However, for the most part the colors look richer and more true-to-life than in the previous edition. For example, if you turn the page and look at the Northern Cardinal, the deeper red looks more like how the species appears in the field than the bright red from the first edition. I included a comparison image below, which also shows the larger size of illustrations in the new edition and some additional text (old edition on the left; new edition on the right).
The maps in the previous edition were also criticized, both for their accuracy and for the sometimes confusing placement of green dots for rare bird sightings. In the new edition, the plague of green dots is gone. The maps now look more like those in the eastern and western regional guides, with gray shading to show patterns of vagrancy. For species with very limited ranges, the maps are zoomed in on the region where they occur, instead of showing the entire continent on every map.
I found a few minor textual errors, but none that would make it harder for readers to identify birds. Hopefully these will be corrected in future printings of this edition. Also, the cover and binding of the second edition feel less sturdy than the first, perhaps a concession to the 50 additional pages in the new edition.
When the Sibley Guide was first published, it set a new standard for field guides to North American birds. I expect the second edition will do the same.