The Red-capped Cardinal forms a superspecies with the geographically separated Yellow-billed Cardinal, P. capitata of the Pantanal, Paraguay and northern Argentina and the Crimson-fronted Cardinal, P. baeri, of Central Brazil.
The distinguishing features of the Red-capped Cardinal are the black bib below the red cap, the generally black bill with a pale base of the mandible, a partial white collar and a black smudge around the eye.
There are illustrations of P.g. gularis of the Amazon Basin in Ridgely & Tudor, Volume 1, Plate 25; and of P.g. nigrogenis of the Orinoco Basin in Hilty & Brown, Plate 55.