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.
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.
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).
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.