Systematics

Geographic Variation

Size (especially tail length) increases and plumage becomes increasingly paler and grayer from east to west, with purported step in these clines at the eastern edge of the Great Plains see Subspecies, below). Also some north to south decrease in size through the Plains.

Subspecies

Two subspecies recognized traditionally (Merriam 1888, Ridgway 1901, Wetmore 1939, A.O.U. 1957, Mengel 1965, Browning 1978):

S. p. pusilla (Wilson, 1810), which includes Fringilla juncorum Nuttall, 1832, and S. agrestis Coues, 1875, is said to breed from central Minnesota east through Ontario to New England and south to e. Texas and east through Florida panhandle (type locality = Philadelphia, Pennsylvania), wintering from the Great Lakes east to Long Island, NY and south to the Gulf Coast and central Florida, and be smaller (male wing < 69 mm, tail < 67 mm; female wing < 64 mm, tail < 63 mm; Ridgway 1901, Pyle 1997); brighter and browner overall than S. p. arenacea, with the gray central crown stripe narrow, rusty post-ocular stripe wider, and black back streaks broad and heavy.

By contrast, S. p. arenacea Chadbourne, 1886, which includes S. p. perissura Oberholser, 1974, and S. p. vernonia Oberholser, 1974 (see Browning 1978), is said to breed in the Great Plains from central Montana and e. Wyoming east to n. North Dakota and south to central Texas (type locality = Laredo, Texas), winter in the s. Great Plains south to ne. Mexico, and be larger (female wing > 65 mm, tail > 65 mm; female wing > 60 mm, tail > 62 mm; Ridgway 1901, Pyle 1997) and paler and grayer overall, with the gray central crown stripe broad, rusty post-ocular stripe narrow, and black back streaks narrow (Merriam 1888, Richmond 1897, Ridgway 1901, Wetmore 1939).

Variation in this species is likely smoothly clinal, however, because there is a broad zone of intermediacy apparently extending hundreds of kilometers from the eastern portion of the Mississippi Valley to the eastern Great Plains; indeed, Wetmore (1939: 240) restricted the range of nominate S. p. pusilla, insofar as it could be diagnosed consistently, to east of a line from s. Quebec through Ohio and central Tennessee to Mississippi. This restriction leaves a zone of intermediacy as wide longitudinally as is the range of S. p. arenacea. Furthermore, birds in central Texas are pale and gray like western birds but small like eastern ones (see “S. p. vernonia,” Oberholser 1974:941), and intermediates have been noted in e. Oklahoma (Sutton 1967:629), w. Kentucky (Mengel 1965:500), and w. Tennessee (Wetmore 1939:240). Richmond (1897:346) noted that “Texan and Mississippi Valley specimens of the Field Sparrow have a tendency to longer wings and tails than the eastern birds, but frequently without any corresponding paleness of plumage. Some of the Texan birds are, however, appreciably paler, but not enough so, and also too small, to refer to arenacea.” Farther east, Ridgway (1901:318) noted that birds west of the Allegheny Mts., Pennsylvania, “average slightly larger, especially in the length of wings and tail, than those east of the mountains, and also very slightly paler in coloration, the variation in both respects being in the direction of S. p. arenacea.”

Wetmore (1939: 241) argued, and Mengel (1965: 500) agreed, that size is too variable to be a consistent diagnostic feature (contra Ridgway 1901, Browning 1978, Pyle 1997). Wetmore (1939: 241) went so far as to observe that “To put the majority of the intergrades with the western form is to place greater emphasis on color than on size, which seems proper, as the size differences separating arenacea from pusilla are minor and the color differences considerable. Color, therefore, is more important than size.”

If subspecies are recognized, placement of intermediates as such would extend the range of S. p. arenacea east to the Mississippi Valley. But as noted above plumage color, too, appears to vary clinally, with a graying trend detectable at least as far east as w. Pennsylvania (Ridgway 1910:318). It therefore appears that subspecies names were applied to polar ends of a smooth cline. Until a detailed quantitative analysis is undertaken, one demonstrating a distinct step in the color or size cline, it seems best to treat the Field Sparrow as monotypic.

Related Species

Worthen’s Sparrow, resident in n.-central Mexico (and formerly sw. U.S.?), is closely related to the Field Sparrow; Sibley and Monroe (1990) classified the two as a superspecies. These species have been treated as conspecific (e.g., Paynter 1970), but primary song differs markedly: the Field Sparrow’s has a “bouncing ball” quality akin to a Black-chinned Sparrow’s (S. atrogularis), whereas Worthen’s Sparrow utters a staccato series of chips akin to the Chipping Sparrow’s (S. passerina). This sharp difference explains Short’s (1966) surprise at encountering a Field Sparrow singing a Chipping Sparrow-like song.

Within Spizella, the Field and Black-chinned Sparrows were found to be sister species by Zink and Dittmann (1993) and Patten and Fugate (1998), although neither study included Worthen’s Sparrow. Most Spizella species appear to have undergone rapid divergence (Zink and Dittmann 1993). No definite hybirds known, although one report of possible hybridization with the Clay-colored Sparrow (S. pallida) in New York (Brooks 1980). The genus Spizella is related to Aimophila and allies (e.g., Chondestes, Amphispiza) (Carson and Spicer 2003), and perhaps to Zonotrichia (Patten and Fugate 1998).