The three subspecies of C. cinereum vary in geographic range, plumage, morphology, and genetics:
fraseri, described as Conirostrum fraseri Sclater 1859; type locality Cuenca, Ecuador.
Subspecies fraseri occurs in the Andes of southern Colombia and of Ecuador. In terms of plumage, it is distinguished by its darker, browner upperparts, its buffier supercilium, and its more ochraceous underparts (Fjeldså and Krabbe 1990).
littorale, described as Conirostrum cinereum littorale Berlepsch and Stolzmann 1896; type locality Peruvian littoral, near Lima and Arequipa (restricted to Lima by Stolzmann and Domaniewski 1927).
Subspecies littorale occurs on the coast and the west slope of the Andes of Peru, south to northern Chile; it also occurs at high elevations in intermontane valleys, such as the Marañón Valley, and perhaps very locally on the east slope of the eastern Andes. The plumage of littorale is similar to that of nominate cinereum, but its supercilium is shorter and its underparts are tinged buff. Subspecies littorale probably is not distinguishable from nominate cinereum in the field.
cinereum, described as Conirostrum cinereum d'Orbigny and Lafresnaye 1838; type locality Yungas, Bolivia.
Subspecies cinereum occurs on the east slope of the Andes of central and southern Peru (northwest at least to Huánuco) and Bolivia (La Paz, Cochabamba, and northern Potosí). See Detailed Description.
The distribution of fraseri may be disjunct from that of littorale (and of cinereum). The southernmost specimen record of fraseri is from Loja, Ecuador (Zimmer 1942d), and recent field observations in Ecuador also only extend as far south as Loja.
The largest of the three subspecies is cinereum, while littorale is the smallest, and fraseri is intermediate but closer in size to cinereum. (Chavez 2016). Body size was assessed by length of the culmen, tarsus, wing chord, tail, and mass from museum specimens in the Museum of Southwestern Biology and the American Museum of Natural History (no mass data are available for fraseri). The only mensural character that does not differ between the three subspecies is culmen length. Subspecies cinereum and fraseri are both larger-bodied, which is likely related to their restriction to humid or semi-humid high-altitude environments, as neither of them occur below ca 2500 meters. This pattern of variation is consistent with Bergmann’s Rule, which predicts that within a broadly distributed taxonomic clade, populations with larger size will be found in colder environments (Bergmann 1847). The subspecies are not known to overlap geographically.
Size and plumage differ between the subspecies, even though culmen size and shape do not. It is therefore likely that these populations are subject to little or no inter-subspecies gene flow, but that they are geographic replacements of one another that inhabit similar, but slightly varying environmental niches. A phylogenetic analysis based on the mitochondrial DNA gene ND2 supports the hypothesis of isolation, placing cinereum in its own monophyletic clade (Chavez 2016). First, Bayesian phylogenetic estimation in BEAST with substantial within-subspecies samplijng (cinereum, n = 27; fraseri, n = 8; littorale, n = 83) revealed monophyly for cinereum, but paraphyly for littorale and fraseri (posterior probabilities of monophyly: cinereum = 1, littorale/fraseri = 1). Second, tests of gene flow and genetic variation support significant differentiation and isolation of subspecies; Fst values between littorale and cinereum (0.738), littorale and fraseri (0.419), and cinereum and fraseri (0.738) are all significantly greater than zero.