Geographical Variation
No quantitative study of geographic variation. No trends in size; although some regional size differences have been suggested, none have proven significant (Miller 1941, Browning 1978), but western individuals may average slightly larger in wing and tail length (see Appendix). Measurements from various localities across range given in Ridgway 1901. Possible association of darker coloration of breeding populations in areas of higher relative humidity (Gloger's rule), but no study has evaluated this. Correlation suggested by differences ascribed to the 2 recognized subspecies (see characters of each below).
Subspecies
Two subspecies recognized (Am. Ornithol. Union 1957); rather weakly differentiated and based on slight differences in coloration and back pattern. Characters said to merge clinally where ranges of subspecies meet (central Great Plains) and to be obscured by individual variation (Pyle 1997). Diagnosis of individuals probably requires comparison to a series of specimens. Characters below from Pyle (1997) and Ridgway (1901).
C. g. grammacus (Say, 1823): Breeds in eastern portion of species' range west to Wisconsin and Minnesota south through e. Nebraska and e. Kansas to ne. Texas; winters from central Texas and s. Louisiana south into Mexico (winter range poorly known; record from Oaxaca needs verification); also recorded from Honduras, Cuba, Nova Scotia, New Brunswick, and several localities southward along Atlantic coast. Blackish streaks on back 2–4 mm wide, chestnut head markings darker (medium-dark rufous), plumage darker.
C. g. strigatus Swainson, 1827: Breeds west of nominate grammacus east to s. Manitoba south to s. Texas and s. Coahuila; winters from southern edge of breeding range south through Mexico, rarely to Guatemala and El Salvador; recorded east to Quebec, New Brunswick, Massachusetts, N. Carolina, and Florida. Black streaks on back narrower (1–3 mm), chestnut markings paler (medium pale-rufous) with less black anteriorly, plumage paler. Included under strigatus as synonyms are C. g. actitus and C. g. quillini. Birds from se. Oregon ("actitus") said to be slightly paler and larger, but no difference found by Miller (1941). Breeders from w. Nebraska south to s. Coahuila said to have shorter wing and tail and intermediate coloration ("quillini") proposed by Oberholser (1974); review by Browning (1978) found coloration of specimens within range of variation for strigatus, and no significant differences in size based on measurements (in mm): mean wing length, male: grammacus, 86.35 (n = 17), strigatus, 86.07 (n = 9), and "quillini," 86.05 (n = 10); mean tail length, male strigatus, 69.7 (n = 6), and male "quillini," 68.7 (n = 6)
