As dictated by the treaty, each nation would send a delegation to the Río de la Plata region to negotiate the border dispute between the Portuguese and Spanish colonies.

Over the course of his time there, he "described 448 birds...This number is reduced to 381 when duplications of sex, age, and plumage are taken into account (eight remain unidentified), and 178 of them are the types upon which the scientific names are based.”[7] He also identified 78 quadrupeds, 43 of which were new.

[3] Before leaving South America, he sent his brother José Nicolás de Azara (then Spanish Ambassador at Paris) his zoological notes and observations, which Médéric Louis Élie Moreau de Saint-Méry published at Paris in 1801 under the title of Essai sur l'histoire naturelle des quadrupèdes du Paraguay.

Azara had largely written his works to correct what he considered to be many errors in Histoire naturelle by Georges-Louis Leclerc, Comte de Buffon.

Darwin also owned a translation of Azara's Essai sur L'Histoire Naturelle de Quadruped du Paraguay.