Azara’s No. 21 “Gavilán pardo obscuro” is not a Short-tailed Hawk Buteo brachyurus

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.

names to his descriptions. In fact, around half of Azara's 448 species were unknown to science at the time, yet his name does not appear as the author of any of them.
Despite the detail in his descriptions, a significant proportion of Azara's descriptions remain either unidentified or have long been identified with the wrong species. Amongst these is his No. 21 Gavilán pardo obscuro, a description that Sonnini (in Azara 1809) considered to be an undescribed species in his French translation of the work (correctly at the time) and for which neither Kaup (1847) nor Hartlaub (1847) were able to propose an identity. The first author to attempt to associate the description with a known species was Berlepsch (1887), who proposed tentatively that it could be the black phase of Short-tailed Hawk Buteo brachyurus and this has been uncritically repeated by subsequent reviewers (Bertoni 1901;Laubmann 1939;Pereyra 1945). However, this identification is erroneous.
Azara's described his No. 21 Gavilán pardo obscuro as follows (my translation, the original text is included in Appendix I): "I have not seen it. Noséda describes it and in the letter in which he sent me its description, he stated that it belongs to this family. Notwithstanding, it seems to approach the falcons, for its gracility and way of flying and for the slender wings and proportions; to the point where perhaps it would be better next to the Swallowtailed Kite Elanus forficatus and not here. My friend's description is as follows.
It is a hawk that is very different from the first species in this family (Great Black Hawk Buteogallus urubitinga and Harris's Hawk Parabuteo unicinctus), because brown dominates in its colour: to the point that upon shooting my rifle it seemed entirely brown and in hand its tones were very dull, similar to those of nocturnal birds. It was flying very high around a burn, circling with many others of its species, mixed with some of a paler pattern and it was clear they were different from the shape and wingspan; because this species had a smoother flight, fast and falcon-like and its movements for catching flying insects that escaped the flames were softer and more graceful. Flying with them were also Black-chested Buzzard-Eagles Geronoaetus melanoleucus, White-tailed Hawks Geronoaetus albicaudatus and the two first swamp hawks (Savanna Hawk Buteogallus meridionalis); all doing the same work and looking for cavies, snakes etc. which are scorched into the open; but of all of them, this one was the most different, smaller, with a strange shape, faster flight, a squarer tail and the wings more pointed, being the same as all others of this species. This was the 22 March and I have never seen another.
Length 18 ¼ inches: tail 7: wingspan 43 ¼. From the bill, where the first feathers are streaked and all the neck, is splashed with dark and smoky-white, almost brown, with this colour running along the edges of the feathers without reaching the tips and dominating in almost all that is visible of them, except for behind the ear and under the ear, where the other is predominant; but the top of the head is all dark brown. The breast and sides follow the same design; but at the beginning of the breast, the paler colour is more perceptible, which in parts is almost cinnamon or to put it another way, the opaque white is between cinnamon and brown, which results in a yellowish colour. The leg has the colour of the neck, but with dark blotches, which form a chain along the middle of the feathers, with the greater underwing coverts the same and the lesser underwing coverts splashed. The belly to the tail is dirty smoky-white, with spots of another colour. The back to the rump and coverts are very dark brown, but the scapular has the edges and some spots of the same yellowish-white of the breast. Some of the median coverts are white and the lesser have the edges white that form an attractive effect; but this pleasant aspect is not complete, because the feathers are old and the most pointed are worn; so that the white edge is missing or lost in most. The flight feathers and tail are dark brown and above, you can just perceive stripes of a stronger colour. Below they are pale, with dark and opaque white stripes.
Flight feathers 24, the third longest. Tail with 12 feathers of even length. Leg 48 lines. Tarsus 32, straw-coloured with hexagonal scales. Mid-toe 18, its claw 7. Bill 11, with a yellow membrane; and the iris is cinnamon-brown. " With the exception of largely dark plumage (present in several species of buteonines that have a dark phase) and small size (which, on the other hand, rules out most Paraguayan species), there is no real evidence supporting the identity of this description as Short-tailed Hawk. The description applies to a moulting juvenile dark phase of Swainson's Hawk Buteo swainsoni. Aside from the fact that the dark phase of the Short-tailed Hawk is extremely rare, there are other glaring inconsistencies between the description and that species, not least the attribution of gregarious behaviour (Short-tailed is largely a solitary bird), the occurrence in an open habitat with other open country hawks (Short-tailed is a forest bird), the description of "gracility" (Short-tailed is compact and fairly robust) and the measurements which are a poor match for this species (Table 1)

(Ferguson-Lees and Christie 2001).
The Short-tailed Hawk is an uncommon resident in Paraguay (Guyra Paraguay 2005). Swainson's Hawk is a Nearctic passage migrant that winters in the pampean region of Argentina (Fuller et al. 1998), with extreme passage dates through Paraguay of 9 September to 14 March (Hayes et al. 1990;Guyra Paraguay 2005). The latter date is remarkably consistent with that reported by Noséda (22 March) and the report of numerous individuals together is consistent with birds on migration. No- séda was a Jesuit priest in the town of San Ignacio, Misiones department in southern Paraguay and this may be assumed to be the locality of his observations, although this was not specifically stated in the text. Note also the "kite" or "falcon-like" impression attributed to the birds by the authors, terms consistent with those used by modern observers when describing the jizz of Swainson's Hawk (Ferguson-Lees and Christie 2001;Clark and Schmit 2017;Wheeler 2018). However, in addition to all of these points (which are consistent with identification as Swainson's Hawk), the following characteristics may be considered to confirm the identification and to rule out Short-tailed Hawk as a candidate species: 1) two tone ventral colouration (versus uniformly dark in Short-tailed Hawk); 2) pale feather edgings on the upper parts (versus uniformly dark in Short-tailed Hawk); 3) dirty white under-tail covers (versus uniformly dark in Short-tailed Hawk); 4) blotchy pattern of under-wing covers (versus uniformly dark in Short-tailed Hawk); and 5) the pattern ascribed to the head (versus uniformly dark with a white forehead in Short-tailed Hawk). Again in all of these cases, the described appearance is broadly consistent with that of a juvenile dark phase Swainson's Hawk. This represents the first report of the species in Paraguay, predating the specimen (MAK 56919) collected by Steinbacher (1962) on 13 December 1955 by over 150 years.