Lower tail-coverts never with markings. No tinge of blue anywhere on the plumage, the ground-color of which is entirely pure white at all ages.
1. Adult. Upper parts, excepting head and neck, with transverse crescentic bars of dark plumbeous; lower parts immaculate, or else without well-defined markings. Young. Upper parts with longitudinal stripes of dark plumbeous; lower parts usually conspicuously striped. Hab. Greenland (in the breeding-season); in winter, occasionally wandering into the northern portions of Europe and North America … var. candicans.
Lower tail-coverts always with markings. A tinge of ashy-blue more or less prevalent above. Young dusky above.
Head and neck above abruptly lighter than the back. Young plain grayish-brown above, with conspicuous whitish borders to the feathers.
2. Adult. Upper parts white, passing into bluish posteriorly; everywhere (except on head and neck) with sharply defined, transverse (not crescentic, but continuous) bars of dark plumbeous. Abdomen and flanks with transverse spots of the same. Young without irregular light mottling to the plumage above, and with broad longitudinal stripes beneath. Hab. Iceland and Southern Greenland, in the breeding-season; in winter, south into Northeastern United States, and Northern Europe … var. islandicus.
Head and neck above abruptly darker than the back. Young (of var. sacer) variegated grayish-brown above, without light borders to the feathers.
3. Adult. Top of the head streaked with whitish; back with sharply defined, continuous, narrow transverse bars, of creamy-white. Hab. Interior regions of Continental Arctic America (Slave Lake, Yukon, and McKenzie River district) … var. sacer.
4. Adult. Top of head not streaked with whitish; back without sharply defined bars of the same. Hab. Continental Arctic Europe (Scandinavia) and Siberia. Migrating south, in winter, to Bengal (Hardwicke) … var. gyrfalco.[48]
b. Lower parts with dusky predominating, or wholly dusky.