Mysticete
Well-known member

It only covers Northern Mexico in so much as as that region is covered is included in the range map. It's reaching to say that there is any intention of covering it by Sibley. Or Nat Geo. Or Peterson.Again, the region is highlighted on the map in the introduction, and Mexico or at least parts of it are often included in people's definition of "North America", which I'm guessing is because the cultural and geographical borders are fuzzy and the Nearctic extends south of the border. So yes, it does appear of pretend to, especially since I haven't found any explanation within the book.
Rather, all of the major guides have used the traditional definition of the ABA territory, which excludes Mexico, and until recently excluded Hawaii. That has always been the geographic focus of US and Canada based birders for listing purposes.