Olivaceous Woodcreeper (Sittasomus griseicapillus) - HBW 8, p. 406

Taxonomy: Dendrocopus griseicapillus Vieillot, 1818, Concepción del Paraguay, Paraguay.
Anatomical characters suggest no close relationship to any other genus. Taxonomy within present species is complex, requiring further research; races probably constitute more than one species, and systematics in need of comprehensive revision. On basis of size and gross plumage coloration, races form five major groups, vocal differences among some of which, when combined with plumage variation, are sufficiently marked to suggest that full biological species may be involved: “griseicapillus group” (including amazonus, axillaris, transitivus and viridis) in Amazonia and Chaco, monotypic “aequatorialis group” on Pacific coast of South America, “griseus group” (including jaliscensis, gracileus, sylvioides, perijanus and tachirensis) in Central America and N South America, monotypic “reiseri group” of NE Brazil, and “sylviellus group” (with olivaceus) of E & SE South America. Last two groups the most likely to represent separate species. Races in in Central America and N South America relatively similar in plumage to, but very different in voice, from Amazonian populations, some of which differ markedly from each other. Geographical variation largely clinal across much of Mexico through Central America to NW South America, with birds becoming smaller and paler to S; birds from N of Isthmus of Tehuantepec (Mexico) named as race harrisoni considered indistinguishable from jaliscensis (both generally larger and greyer than birds farther S), although intermediates between harrisoni and sylvioides reported from Puebla; forms synonymized with sylvioides, which also intergrades with gracileus in S Belize, have been described from W Panama S to N Colombia (levis), E Panama (veraguensis) and Colombia (enochrus), where numerous intermediates occur and marked individual variation apparent. Proposed race viridior, from E Bolivia, hardly distinguishable from and considered better merged with viridis, perhaps reflecting intergradation between latter and nominate. Fifteen subspecies recognized.