I don't know whether this has come up here before, but after posting trip reports recently, it struck me again. BF - and many others - consider Mexico to be part of North America. But going through these "sticky threads", it seems clear that Mexico belongs to Central America in the opinion of those who discuss the topic of FGs.
Howell and Webb tell you that the major change in Avifauna happens at the isthmus of Tehuantepec. However, Andy Kratter here in BF has argued that the change in Avifauna at the Rio Grande (but going north of the sky-islands of SE Arizona) is a stronger change than the change in Avifauna at the isthmus of Tehuantepec. The areas of Yucatan certainly do not induce a strong change in Avifauna at the border of Mexico and it's southern neighbors. Therefore, areas East and South of the Isthmus are certainly Central America, and the rest of Mexico you can go with who you believe in