Comparison of previous descriptions of plumage and soft-part characteristics with specimens and recent observations of the species in southern Peru suggests that there may be some (as yet undocumented) geographical variation. Iris color throughout its distribution has been referred to as buffy brown, brown, orange rufous, 'pardo', coffee, reddish, red, or pale-chestnut (Dabbene 1926, Swales 1926, Remsen et al. 1982, Lloyd et al. 2005). Tarsus color previously has been described as silvery-flesh, flesh, light brown, or brown (Cassin 1866, Lloyd et al. 2005, Pearman in prep). See also Priorities for Future Research.
The Short-tailed Finch has a somewhat ‘dynamic’ taxonomic history. When the species was formerly classified some years after it had been first collected, it was originally considered to be part of the family Icteridae. Cassin (1866) described the species as "evidently Icterine, and allied to Quiscalus and Scolecophagus, but not to be referred with any considerable degree of propriety to either of those or to any genus of this group". Other authors have referred to the species as being "originally placed amongst the icterids, somewhere between the genera Quiscalus and Scolecophagus " (e.g. Dabbene 1926). Sclater (1884) reexamined specimens and subsequently placed the species amongst the Fringillidae. Paynter (1970) suggested that the genus Idiopsar could be merged with Diuca (Remsen et al. 2009). Monotypic, the Short-tailed Finch is currently classified among the Embezeridae (Remsen et al. 2008) and thought to be more closely related to the Phrygilus sierra-finches (Ridgely and Tudor 1989, Remsen et al. 2009).
The above summary, taxonomy, and detailed description of the identification of Short-tailed Finch draws on the discussions in standard field guides and other recent literature: Ridgely and Tudor (1989), Fjeldså and Krabbe (1990), Lloyd et al. (2005), and Schulenberg et al. (2007).