Geographic variation in Antrotomus ridgwayi is "weak and probably clinal" (Pyle 1997). Two subspecies usually recognized (Peters 1940, Bowers and Dunning 1997, Dickinson and Remsen 2013):
ridgwayi, described as Antrostomus ridgwayi Nelson 1897; type locality Tlalkisala, Guerrero, Mexico
Occurs in the extreme southwestern United States and in Mexico.
See Detailed Description.
troglodytes, described as Caprimulgus ridgwayi minor (Griscom 1929); type locality Progreso, Guatemala; the name minor was preoccupied, hence renamed Caprimulgus ridgwayi troglodytes (Griscom 1930)
Occurs in Guatemala and Honduras.
Similar to nominate ridgwayi, but "very radically smaller; wing, 148 instead of 161; tail, 93 instead of 115; exposed culmen, 11.5 versus 15.5" (Griscom 1929: 10; these measurements are of the holotype, a male).
Although ridgwayi was described as a species of Antrostomus, this species later was transferred to Caprimulgus, a genus that eventually encompassed a large number of species of nightjars worldwide (Salvin and Hartert 1892, Peters 1940, Dickinson 2003). Phylogenetic analysis of DNA sequence data, from both mitochondrial and nuclear genes, reveals that the broadly defined Caprimulgus of Peters (1940) and other authors is highly polyphyletic (Han et al. 2010). Caprimulgus proper is entirely confined to the Old World, and New World species of "Caprimulgus" are split into several clades. Ridgwayi belongs to a clade that are restored to Antrostomus. Within Antrostomus, ridgwayi is basal to a clade that also includes Antrostomus salvini (Tawny-collared Nightjar), Antrostomus rufus (Rufous Nightjar), and Antrostomus carolinensis (Chuck-will's-widow) (Han et al. 2010).