American Robin

Turdus migratorius



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Geographic Variation

Figure 3. Relationship between size and climate in the American Robin.

The association between median wing length in male robins and mean noon values for climate variables in July for 21 ecoregions in the United States. Larger circles have larger robins; ecoregions in the East have solid circles and those in the West have open circles. Note that although dry-bulb temperature is negatively related to size variation in the east, wet-bulb temperature and absolute humidity are better predictors overall. From Aldrich and James (1991). Copyright Allen Press, used with permission.

Plumage is darkest in the cooler Pacific Northwest and Newfoundland, two places where relative humidity (percent saturation of the air), not absolute humidity, is exceptionally high (Aldrich and James 1991), a pattern consistent with Gloger's Rule (Gloger 1833, Grinnell 1910e; cf. Mayr 1963). The amount of white in the tail decreases progressively from east to west across the United States, reaching its nadir in the Pacific coastal forests. An isolated population resident in the mountains of Baja California Sur is exceptionally pale overall.


Geographic variation is rather limited in American Robin, with birds from western regions averaging palest overall and with less white in the outer tail feathers. Subspecific field identification is difficult due to the large degree of individual variation.

© Brian Sullivan, California, United States, 3 February 2007

Size variation is only moderate across the species' wide breeding distribution. Range of median wing length in adult females from 39 ecoregions was 119.1–133.0 mm (n = 343) and 124.7–137.2 in adult males (n = 606) from 36 ecoregions (Aldrich and James 1991). The overall pattern of geographic variation in size on breeding grounds is not a direct function of latitude, contra Bergmann's Rule (Mayr 1963), but it is predictable from statistics related to variables that are a function of both temperature and humidity: average wet-bulb temperature and absolute humidity in July have high predictive power (Figure 3; Aldrich and James 1991, James 1991a). Robins are smallest in the low, humid southeastern U.S. and are small along the humid coast of northern California and the Pacific Northwest. They are largest in the high, dry Rocky Mountains, northern Great Plains, and northern deserts of the U.S. (Aldrich and James 1991), where evaporation often exceeds precipitation.

Bill length covaries with general size, except that birds with the proportionally longest bills belong to the isolated population in Baja California Sur. Leg length as a proportion of general size is small in Mexico and large in birds of the cool humid forests along the northern Pacific Coast. Tarsi are longer in the cool maritime forests of Newfoundland and adjacent areas, but they are exceptionally short in Baja California Sur. Wing shape becomes more pointed in larger birds, but is even more pointed in the Arctic than would be predicted by this trend. Heart and lung masses increase with altitude (Dunson 1965).


Seven subspecies are listed below, following Phillips 1991, and organized into two groups (American Ornithologists' Union 1998a). Subspecies are diagnosed on the basis of variation in body size and plumage color. Recognition of these subspecies by Phillips 1991 is based on former work by multiple authorities (Baird 1864, Ridgway 1907, Grinnell et al. 1909, Swarth 1912b, Oberholser 1917dOberholser 1974, Aldrich and Nutt 1939Rand 1948aRand 1948b, Webster 1959bTodd 1963aMengel 1965bSutton 1967bBrowning 1974bBrowning 1990Godfrey 1986, Cramp 1988). See Pyle 1997c for wing and tail measurements by subspecies in the U.S. and Canada and Aldrich and James 1991 for measurements by ecoregions.


American Robins from Atlantic Canada and the Pacific Northwest average darkest overall, conforming with Gloger's Rule. This individual is possibly of the Atlantic Canada breeding subspecies, T. m. nigrideus, based on its blackish upper back showing little contrast with the black nape and head. However, individual variation makes subspecific identification tentative outside known breeding areas.

© Ian Davies, Massachusetts, United States, 8 April 2013

Formerly treated as a distinct species, the San Lucas Robin (T. m. confinis) is a resident subspecies in the dry mountains of Sierra Victoria and other mountains in Baja California Sur. Note the pale overall appearance, smokey gray upperparts, creamy buff breast, and complete supercilium.

© Pam Rasmussen, Baja California Sur, Mexico, 6 March 2016

American Robin (migratorius group):

T. m. migratorius Linnaeus 1766, includes T. canadensis Müller 1776. Largest breeding range (Hellmayr 1934, American Ornithologists' Union 1957, Ripley 1964b), extending from northern edge of treeline from northwestern Alaska east to southern Québec south to southern Alaska, central British Columbia, and southwestern Kansas east to central Alberta, Pennsylvania, and New Jersey; overwinters from Idaho, central Great Plains, Ohio Valley, southern Great Lakes region, and southern New England south through Mexican Plateau [type locality = South Carolina]; vagrants collected west to Pribilof Islands and California and south to Yucatán (Phillips 1991). Mantle slate gray to brownish gray; breast orange-red; throat streaks fine; white on outer rectrix >4 mm and usually >9 mm; supercilium discontinuous; breast of juvenile cinnamon to tawny or buffy, spotted heavily.

T. m. nigrideus Aldrich and Nutt 1939. Breeds from northern and central Québec (perhaps Manitoba; Rand 1948a) east through Labrador to Newfoundland [type locality = Avalon Peninsula, Newfoundland]; overwinters from Maritime Provinces west to Great Lakes (Aldrich 1945a) and south to northern Florida and Mississippi; vagrant to Greenland and Louisiana. Similar to T. m. migratorius but mantle blackish, breast deep rufous, and throat streaks thick and coalescent.

T. m. achrusterus Batchelder 1900. Largely resident from Ohio Valley east through West Virginia to Atlantic Coast and south to central Texas and Florida [type locality = Raleigh, North Carolina], with breeding range extending southward for several decades (Odum and Burleigh 1946, Stevenson and Anderson 1994b; see Distribution: historical changes, above); overwinters in the southeastern states; vagrant to Isla Holbox, Yucatán (Phillips 1991). Like T. m. migratorius but paler and grayer (less brown) dorsally and paler and tawnier (less rufescent) ventrally; averages smaller; breast of juvenile whitish, spotted modestly.

T. m. caurinus Grinnell 1909. Breeds on islands from Glacier Bay, Alaska, south to Vancouver and on Olympic Peninsula, Washington, and, perhaps, northwestern Oregon (Phillips 1991) [type locality = Admiralty Island, Alaska]; overwinters from southwestern British Columbia (occasionally southern Alaska) south to central coastal California and inland to Idaho; vagrant to southern California and central Arizona. Like T. m. migratorius but white on outer rectrix <8 mm and typically <4 mm; darker overall than T. m. propinquus.

T. m. propinquus Ridgway 1877, includes T. m. aleucus Oberholser 1974 (Browning 1974b). Breeds from southern British Columbia east to southwestern Saskatchewan south to southern California and northern Baja California and through Rocky Mountains to western Zacatecas, Guanajuato, and southwestern Nuevo León [type locality = Laramie Peak, Wyoming]; partially migratory, with many overwintering south to Baja California, Sonora, Arizona, southern Great Plains, and central Texas. Like T. m. migratorius but paler overall, averages larger; white on outer rectrix <4 mm. Breast duller (less ochraceous) and darker than T. m. phillipsi.

T. m. phillipsi Bangs 1915, includes T. m. permixtus Griscom 1934. Resident in highlands of Jalisco, Guanajuato, and southern Hidalgo south to southern Oaxaca [type locality = Las Vigas, Veracruz]. Like T. m. propinquus but darker dorsally, more ochraceous (less red) ventrally.

San Lucas Robin (confinis group):

T. m. confinis Baird 1864. Resident in the dry mountains of Sierra Victoria and other mountains in the Cape region of Baja California Sur [type locality = Todos Santos, Baja California Sur]. Pale overall; mantle smoke gray; breast creamy buff; supercilium complete. Formerly treated as a distinct species (American Ornithologists' Union 1957, Phillips 1991:56).

Related Species

The family Turdidae (thrushes) occurs on every continent, with exception of Antartica. It is characterized by having the juvenile plumage spotted (usually), a single annual molt, and an undivided (booted) tarsus (except lower portion), rather than a scutellate horny covering over the tarsus. The family is a member of superfamily Muscicapoidea (Ericson and Johansson 2003, Ericson et al. 2003a) along with the Mimidae (thrashers), Sturnidae (starlings), Cinclidae (dippers), and Muscicapidae (Old World flycatchers).

The diverse genus Turdus, found on every continent except Australia, where nonetheless some species in the genus have been introduced successfully, has a wide array of plumage patterns but rather modest differentiation in morphology (size and shape overall and of bill). Sixteen species of Turdus thrushes reside in North and Middle America. Of these, eight have their regular northern limit in Mexico, the West Indies, or Costa Rica and Panama, and two in Costa Rica and Panama (American Ornithologists' Union 1998a). The American Robin is the only member of the genus that occurs throughout the North American continent.

In terms of morphology and song, the species most similar to T. migratorius is T. rufitorques, the Rufous-collared Robin of subtropical montane forests, temperate pine-oak woodlands, and highland brushy associations from Chiapas to El Salvador and western Honduras (Hellmayr 1934, Phillips 1991:57, Marshall 2001). The two may form a superspecies (American Ornithologists' Union 1998a), and a recent molecular phylogeny (Voelker et al. 2007) recovered a sister-species relationship between them.

Recommended Citation

Vanderhoff, N., P. Pyle, M. A. Patten, R. Sallabanks, and F. C. James (2016). American Robin (Turdus migratorius), version 2.0. In The Birds of North America (P. G. Rodewald, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA.