ARCTIC WARBLER (Phylloscopus borealis) is very similar to Greenish Warbler (Phylloscopus trochiloides), but averages larger, stouter, and more attenuated, with a relatively larger head. It has a flatter crown and proportionally longer wings (with slightly longer primary projection) and a shorter tail. But as the two species are very unlikely to be seen together in most of the regions these differences are hard to compare.
Also, the differences in calls and songs are diagnostic. The specific identity of non-calling birds is problematic but concentration on a combination of features should resolve the issue. The Arctic typically has conspicuously pale pinkish or straw-colored legs. Whereas Greenish typically has darker, grayer legs, although many Greenish, especially 1st-year birds in autumn, have grayish-pink legs that appear very pale in some lights.
The large pale bill of the Arctic has often been quoted as a useful field feature, but the individual variation makes this invalid. Greenish warbler typically has a stronger bill than the more familiar Chiff-chaff and Willow Warbler, and in fact, frequently has more extensive pale on the bill than the Arctic. The latter usually has a dark tip to the lower mandible, while many Greenish lacks a dark tip or show only a dusky smudge, but there is much overlap.
Nonetheless, the bill base is deeper in the many Arctics and this creates a distinctly large- and heavy-billed impression in such individuals. The supercilium of the Arctic is brighter and clearer and is exceptionally long, extending further behind the eye than on Greenish. And, tapering towards the nape; typically, supercilium of Greenish broadens markedly and ends quite squarely midway between the eye and the nape. But some birds have a longer, tapering supercilium recalling the Arctic.
More important is the front of the head: the supercilia meet the base of the bill in Greenish, narrowly joining on the lower forehead in some individuals. However, in the Arctic, the supercilia begin abruptly just before the base of the bill so that the forehead and anterior lores are wholly dark.
This dark olive area then extends widely through the eye and borders the lower part of the supercilium and is distinctly darker than the crown. Whereas in Greenish the dark eye stripe is not obviously darker than the crown and is diffuse on the anterior lore, starting clearly as a dusky spot in front of the eye.
The ear coverts of the Arctic tend to be more strongly mottled with olive than in Greenish, which shows cleaner ‘cheeks’ in fresh plumage. The wing pattern is similar in the two species, although the Arctic in fresh plumage has a strong tendency to show a second wing bar, on the median coverts (rare in viridanus race of Greenish but frequent also in southern nitidus and vagrant plumbeitarsus). The tertials are uncommonly lacking in contrast, but Greenish is often very similar with the centers only a shade darker than the fringes.
The underparts of the Arctic are more sullied with gray than on fresh Greenish (of races viridanus and plumbeitarsus), especially on the breast and flanks, but both can show a pale yellowish suffusion (but never as strong as in nitidus race of Greenish). The vagrant eastern race plumbeitarsus of Greenish (‘Two-barred Greenish’).
Which also tends to have darker gray legs than the other races of Greenish and broader wing bars than in the Arctic. Hence, so is less likely to be confused. The Arctic is very active when feeding, moving quickly through the foliage, darting between perches with less wing- and tail-flicking than Greenish.
Arctic Warbler flight action is between trees and bushes stronger, and less fluttery, than Greenish. On the other hand, the 1st primary is no more than 3 mm longer than primary coverts (5–10 mm longer in Greenish) and the Arctic lacks imagination on the 5th primary shown by Greenish.
Males average larger and longer-winged than females. 1st-year birds in autumn are in fresh plumage, whereas adults have wings and tail abraded (although less markedly so than in adult Greenish). The 1st-year birds also tend to be brighter and may show a weak yellowish suffusion to underparts, which are typically drabber in adults.
Arctic Warbler call is quite distinctive and readily and repeatedly uttered, a sharp, almost metallic ‘dzik’ or ‘dzrt’. It is recalling a sharper version of the call of White-throated Dipper. Arctic Warbler song is a characteristic, far-carrying, monotonous but fairly melodious rattle, recalling the main song phrase of Wood Lark with a touch of Lesser Whitethroat in tonal quality.
The bird can be rendered ‘dyryryryryryryryry’ recalls Bonelli’s Warbler to some extent, but the rattle is longer (about three seconds) and often changes abruptly in pitch. The periods between ‘rattles’ are usually interspersed with distinctive call notes.
Arctic Warbler is scarce in N Scandinavia, but fairly common further east. However, the winters are in SE Asia. Arctic Warbler breeds in birch or fairly open coniferous woodland, frequently on slopes, and in willow scrub in the northern ‘taiga’ zone and at the edge of the tundra; frequently near water. Migrants frequent both wooded and bushy areas.