
These are species with at least one recorded territory in Mexico, not a national checklist: a bird list from eBird or Avibase is more complete. The counts follow recording effort as much as wildlife, so a country with more visitors and more naturalists will show more of both.
The species with the most open occurrence records in Mexico, among those a photograph exists for. Most recorded is not most numerous: it rewards animals that are common, conspicuous and live where people go.

Quiscalus mexicanus

Melanerpes aurifrons

Pitangus sulphuratus

Columbina inca

Zenaida asiatica

Cyanocorax yncas

Mimus polyglottos

Ortalis vetula

Dryobates scalaris

Cathartes aura

Icterus gularis

Agelaius phoeniceus
The reference database has no regions filed under Mexico.
The field guide covers Mexico on the map, but none of its species have an entry written for them yet.