[order] Passeriformes | [family] Sylviidae | [latin] Phylloscopus borealis | [UK] Arctic Warbler | [FR] Pouillot boréal | [DE] Wanderlaubsänger | [ES] Mosquitero boreal | [IT] Luì boreale | [NL] Noordse Boszanger

Noordse Boszanger determination

copyright: K. Blomerley

Medium-sized, rather strong-billed, fairly slim yet robust, and highly active Phylloscopus; looks as bulky in flight as Wood Warbler. Plumage bright olive-green above and grey-white below, with orange-yellow base to strong bill, long, often upturned white supercilium, prominent white eye-crescents interrupting dark eye-stripe, 1 distinct and 1 indistinct white wing-bar (in fresh plumage), and bright straw legs and feet. Call distinctive.

Might more accurately be named Subarctic Warbler, since breeding range falls entirely within that region, and as a tree-dweller is precluded from living beyond July isotherm of 10°C, beyond which tree growth is inhibited. Remarkable also for combining successful occupancy of so much of Holarctic taiga zone with absence from forests south of boreal climates except in east Asian mountain regions. Despite inhabiting largely coniferous forests, shows marked preference for birch, poplar, willow, and other broad-leaved trees or scrub wherever available, especially along river banks and near water, but in places also frequents dry, sparsely wooded slopes and even rhododendron thickets.

Phylloscopus borealis is a widespread summer visitor to the boreal zone of Fennoscandia and Russia, with Europe accounting for less than a quarter of its global breeding range. Its European breeding population is very large (>4,500,000 pairs), and was stable between 1970-1990. No trend data were available for the stronghold population in Russia during 1990-2000, but the small populations in Norway, Sweden and Finland were all stable.

Arctic Warblers most commonly (almost solely) feed on insects.Arctic Warblers obtain most of their food by gleaning dwarf birch or willow shrubs. Hawking from the shrub layer is not uncommon. Occasionally, these forays reaches heights of 10 m with the birds turning a somersault at the peak of flight after capturing a moth or Dipterid. This impressive display may be a territorial or breeding behavior, although such behavior is observed well into the nestling period when there is relatively little singing, chasing, or other evident mating behaviors. Arctic Warblers are also observed to feed while hovering, particularly near the crowns of spruce.

This species has a large range, with an estimated global Extent of Occurrence of 10,000,000 km². It has a large global population estimated to be 27,000,000 individuals (Rich et al. 2003). Global population trends have not been quantified, but the species is not believed to approach the thresholds for the population decline criterion of the IUCN Red List (i.e. declining more than 30% in ten years or three generations). For these reasons, the species is evaluated as Least Concern. [conservation status from birdlife.org]

In Northern Russia laying begins late June, in Northern Sweden first eggs are laid in last week of June or first week of July. Nest is built on ground in vegetation, or in natural crevice in mossy bank or among tree roots, often well concealed. Nest is a domed structure with side entrance, of dry thin grass stems and leaves, often with moss and dry leaves, lined with finer grasses, but only rarely with feathers or hair. Clutch size is 5-7 eggs which are incubated for 11-13 days. Young fledge 13-14 days after hatching.

All populations migratory, wintering in southern south-east Asia: widespread and generally common from Tenasserim (southern Burma) and (chiefly southern) Thailand east to Philippines and Moluccas, south to Greater and Lesser Sundas, and in Taiwan. Western populations migrate east across Russia, gradually turning southward east of Yenisey through eastern Mongolia and Manchuria to reach winter quarters via eastern China; route thus exceptionally long, at least 13 000 km for Fenno-Scandian birds. From Fenno-Scandia and north-west Russia, autumn departures chiefly in August. Birds arrive Malay peninsula from mid-September. Leaves winter quarters late, mostly April to early May, arriving in west Palearctic breeding grounds mostly in 2nd half of June. Regular autumn vagrant to Britain in small numbers, with distinct peak mid-September; probably resulting mostly from reversed migration. Elsewhere in Europe, vagrancy rare but apparently increasing.