The Willow Warbler (Phylloscopus trochilus) is Sweden's most common bird, a small, yellow-green songbird whose wistful, descending song fills every Swedish deciduous wood from May to July. It measures 11–12.5 cm and weighs just 7–15 g, with olive-green upperparts, a yellowish-white throat and breast, a strongly marked pale yellow eyebrow stripe, and a thin, pointed bill. Its pale, brownish-pink legs are the easiest way to distinguish it from the very similar Chiffchaff, which has dark legs.
Its song is a mild, clear series of notes falling steadily in pitch — "si-si-su-su-so-so-se-se" — quite unlike the Chiffchaff's monotonous "chiff-chaff," while the call is a soft, two-syllable "hu-it" with the stress on the first syllable. The Willow Warbler forages restlessly through foliage for insects and spiders, occasionally sallying after flying prey, and takes berries in autumn. It nests on or near the ground, well hidden and often sheltered by a hanging branch, building an almost spherical, domed structure of coarse grass and moss lined with feathers; most pairs lay 4–8 eggs in early June, with the female incubating for 12–14 days and both parents feeding the young, who fledge after 11–15 days, often followed by a second brood. Breeding across the whole of Sweden from Skåne to the mountain birch forest in the north, the species comprises two subspecies whose ranges meet around a line from northern Hälsingland to northernmost Dalarna and which take different migration routes to Africa — southern birds heading southwest to West Africa, northern birds southeast to eastern and southern Africa. The Swedish population was estimated in 2018 at more than 13.2 million pairs, making the Willow Warbler by far the country's most numerous breeding bird.
Read about this species in Swedish →