The Florentine Codex is a Renaissance-era illuminated manuscript that contains the earliest-known regional work on the birds of México. Its Nahuatl language texts and scholia (the latter later incorporated into its Spanish texts) were written in the 1560s by Bernardino de Sahagún’s research group of elite native Mexican scholars in collaboration with Aztecs from two cities: Tlatelolco and Tenochtitlan. In the present study, I compared the contributions from these two cities and found many differences. While both cities contributed accounts and descriptions of land and water birds, those from Tlatelolco were mainly land birds, while those from Tenochtitlan were mainly water birds. Tlatelolco contributed over twice as many bird accounts as Tenochtitlan, and supplied the only information about medicinal uses of birds. Tenochtitlan peer reviewed the Tlatelolco bird accounts and improved many of them. In addition, Tenochtitlan contributed all information on bird abundance and most information about which birds were eaten and not eaten by humans. Spanish bird names appear more frequently in the Aztec language texts from Tenochtitlan. Content analysis of the Tenochtitlan accounts suggests collaboration with the water folk Atlaca (a prehistoric lacustrine culture) and indigenous contacts with Spanish falconers. The Renaissance-era studies of Sahagún’s research group, on a now lost island in the formerly vast, bird-rich wetlands of the Valley of México, constitute the birth of Mexican ornithology and, coincidently, give the history of Mexican ornithology a distinctive, Aztlán-like beginning, significantly different from the ornithological histories of neighboring countries.