(872) Acanthopneuste borealis borealis.
The Arctic Willow-Warbler.
Phyllopneuste borealis Blas., Naum., 1858, p. 313 (Lake Ochotska). Acanthopneuste borealis. Blanf. & Oates, i, p. 412.
Vernacular names. None recorded.
Description. Whole upper plumage, including exposed parts of wings and tail olive-green, a little lighter on the rump; wing-coverts tipped with yellowish white forming two wing-bars, conspicuous in fresh plumage but inconspicuous in abraded plumage, the tips to the median coverts being often completely worn away; a well-defined supercilium from the nostrils to the nape yellowish white sometimes tinged with buff; lores and a line behind the eye dark brown ; ear-coverts olive-brown, mottled with yellow; under plumage yellowish white, deeper yellow in Winter than in Summer; flanks generally suffused with dusky; under wing-coverts and axillaries pale yellow.
Colours of soft parts. Iris hazel to dark brown; bill, above dark brown tipped yellowish, below paler, the basal half almost orange-yellow; mouth bright orange-yellow; legs and feet flesh-colour or yellowish-fleshy.
Measurements. Total length about 130 mm.; wing 64 to 72 mm.; tail 45 to 51 mm.; tarsus about 19 to 21 mm.; culmen 11 to 12 mm.; first primary 9 to 11 mm.
Distribution. Breeding in Norway, North Russia and Northern Asia to Dauria, Transbaikalia, Corea, Ussuri-land to Kamchatka. In Winter South to Assam, Burma, Andamans, the Indo-Chinese countries and South China, etc.
Nidification. Breeds during June, making a domed, or semi-domed, nest of moss and dried grass lined with finer grass and placed on the ground in deciduous forests. The eggs number five to eight, and are just like rather large eggs of the Common Willow-Warbler. The ground-colour is pure white with numerous small specks of light reddish scattered over the whole surface. Hartert, quoting Pleske, gives the average size as 15.3 X 12.3 mm.; maxima 16.0 X 12.5 mm.; minima 15.0 X 12.0 mm.
Habits. Similar to those of the Willow-Warbler.