The Arctic Warbler (Phylloscopus borealis) is a leaf warbler of Sweden's mountain birch forests, 11–13 cm long with a wingspan of roughly 18–22 cm and a weight of only 8–13 grams — noticeably larger and heavier-billed than the smaller warblers it shares habitat with. It has olive-green upperparts and whitish underparts, with a long, clear pale supercilium (eyebrow stripe) and a distinct white wingbar as its best field marks; a fainter second wingbar is sometimes visible. Its relatively heavy bill and bulkier build set it apart from the similar Wood Warbler.
The species breeds from northernmost Europe eastward through Russia to Alaska, and Sweden holds its westernmost outpost — only a handful of pairs nest in the far north, in mountain birch forest with willow scrub. Though classed as globally secure (LC), the tiny and isolated Swedish population makes it Endangered (EN) on the national red list. It arrives late, not until mid-June, and departs again by August to winter in Southeast Asia, leaving little time for breeding: the nest is a domed structure with a side entrance built on the ground and hidden in vegetation, with clutches of 5–7 eggs. It feeds on insects and spiders gleaned from leaves and twigs in birch and willow growth. The song is a fast, trilling, evenly paced series rendered as "tsississississi," and the call is a hard "tsick" — both best heard in June and July in the Lapland mountains, where the species can be genuinely difficult to detect except by voice.
Read about this species in Swedish →