On of the best birding places I've been to is the Yucatan. The list for the Yucatan will be done shortly the Mexican list is at http://worldbirds.awardspace.com/Mexican_Birds.htm
It is not only a good place for birding, the history and culture of the place is fabulous.
One of the best spots is Rio Lagartas if you walk along the front you will find a boat and guide to take you around the lagoon. See video http://www.youtube.com/watch?v=SEMs7JqMw_A Not a good video but it shows just how prolific it is. My favotite bird there was the Boat_billed Herons.