Almost half of the 400+ species described in Félix de Azara’s “Apuntamientos para la historia natural de los páxaros del Paraguay y Rio de la Plata” were new to science at the time of its publication; however, not all of these have yet been correctly identified. Azara’s No. 21 “Gavilán pardo obscuro” has long been misidentified as a dark phase of the Short-tailed Hawk Buteo brachyurus. However, the description of plumage, jizz, habitat and measurements are inconsistent with that species and Azara’s No. 21 can, in fact, be convincingly identified as a juvenile Swainson’s Hawk Buteo swainsoni, the earliest Paraguayan report of this species. No scientific names were apparently ever based on Azara No. 21.