Between 1783 and 1803, Martín de Sessé and José Mariano Mociño led the Real Expedición Botánica a Nueva España and explored the major part of the territory. Upon their return to Spain, towards the end of 1803, they began to reorganize the materials the expedition had collected, trying to gather together all the paintings, manuscripts and specimens in order to publish them. Some material was published during the nineteenth and twentieth centuries. But the “Descripción de distintas especies de aves del Reino de Nueva España, según sus órdenes y familias” remained unknown until 1998, when it was discovered at the Museo Nacional de Ciencias Naturales in Madrid. This manuscript is probably the expedition's major contribution to the field of zoology. In this article, we attempt to look at the context in which this manuscript was written. We hope to reconstruct the events of the expedition, as well as to assess its potential scientific contribution, despite its being unknown to the scientific establishment of the time.