Red-necked Woodpecker is polytypic, with three subspecies that vary slightly in size and plumage. The following descriptions are from Winkler et al. (1995) and Short (1982).
Nominate C. r. rubricollis in the northern part of range in the Guianas, Colombia, and Venzuela to Ecuador and the north bank of the Amazon River. Described above. It is the smallest subspecies and tends to have pale rufous underparts. Intergrades with trachelopyrus in southwest of range (i.e., Ecuador, northern Peru and northwestern Brazil).
C. r. trachelopyrus from northeast Peru to La Paz, Bolivia. Large and darker, more chestnut below than nominate. Red feather tips extending to lower breast or belly. Rufous wing patch that extends to outer vanes of the outer primaries.
C.r. olallae south of the Amazon, from Madeira River to Pará and Maranhão in Brazil and south to Cochabamba in Bolivia. Intermediate in size between the other two species. Resembles trachelopyrus, but brighter in red and rufous colors.
The name Campephilus ("lover of grubs") refers to the diet of this group that feed mainly on beetle larvae, by George Robert Gray in 1840, an English zoologist and author. C. rubricollis, nominate Red-necked Woodpecker species, was named in 1783 by Pieter Boddaert, a Dutch physician and naturalist. Subspecies C. r. trachelopyrus was named in 1857 by Alfred Malherbe, a French magistrate and naturalist. Subspecies C. r. olallae named in 1945 by N. Glydenstolpe, a Swede (Zoonomen 2011).
Fleischer et al. (2006) reported the first molecular phylogeny for the genus Campephilus, based on mitochondrial DNA sequence data. This survey, which omitted two species, Robust Woodpecker (Campephilus robustus) and Magellanic Woodpecker (Campephilus magellanicus), places Red-necked Woodpecker in a clade with one sister species, the Cream-backed Woodpecker (Campephilus leucopogon). The basal member in the genus is the Crimson-bellied Woodpecker (C. haematogaster).
The 11 member species of this New World genus are similar in coloration, habits, and tail and foot structure (Short 1970, 1982 p. 432). It seems that the greatest diversification of the Campephilus genus occurred in South America after the lineage ancestor entered South America across the Isthmus of Panama (Moore et al. 2011). The estimated divergence date between two sister clades comprising the northern ivory-bills and other Campephilus, occurred 3.2 MYBP (Moore et al. 2011).