Coloration of the sexes alike at all ages; old and young slightly different in pattern and tints. Scutellæ of tarsus and toes uninterrupted from “knees” to claws; tarsus but little longer than middle toe. Bill large, the cere on top about one third the culmen. Size medium; form very slender … _Rhynchofalco._ SUBGENUS HIEROFALCO, CUVIER. _Hierofalco_, CUVIER, 1817. (Type, _Falco gyrfalco_, LINN.) _Jerafalco_, BOIE, 1822; KAUP, 1851. (Same type.) _Gennaia_, KAUP, 1847. (Type, _Falco jugger_, GRAY.) Species and Races. =1.= =F. gyrfalco.= Wing, 13.00–17.00; tail, 8.50–11.50; culmen, .85–1.05; tarsus, 2.10–3.00; middle toe, 1.80–2.25.[45] Ground-color varying from entirely pure white to wholly dusky, but generally bluish (in adult) or grayish-brown (in young) above, and white beneath. _Adult._ All the markings transverse.[46] No lighter nuchal band. _Young._ Markings of the lower surface longitudinal, the upper parts without transverse bars (except on the tail[47]). _a._ Lower parts with white predominating, or wholly white. 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.