The willow warbler (Phylloscopus trochilus) is a very common and widespread leaf warbler which breeds throughout northern and temperate Europe and the Palearctic, from Ireland east to the Anadyr River basin in eastern Siberia. It is strongly migratory, with almost all of the population wintering in sub-Saharan Africa.
It is a bird of open woodlands with trees and ground cover for nesting, including most importantly birch, alder, and willow habitats. The nest is usually built in close contact with the ground, often in low vegetation. Like most Old World warblers (Sylviidae), this small passerine is insectivorous. The willow warbler is now one of around 80 species placed in the genus Phylloscopus that was introduced by the German zoologist Friedrich Boie in 1826. The genus name combines the Ancient Greek meaning "leaf" and ' meaning "seeker" (from , "to watch"). The specific epithet is Ancient Greek meaning "wren".
Before the English name was standardised to willow warbler by William Yarrell in 1843, it alongside the chiffchaff and wood warbler were considered to be a single bird known as the "willow wren".
Three subspecies are recognised.
Behaviour
All populations are highly migratory, with the subspecies P. t. yakutensis migrating up to from eastern Siberia to southern Africa along the Asian–East African Flyway, one of the longest migrations of any for a bird of its size.
Status and conservation
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thumb|Egg, Collection [[Museum Wiesbaden, Germany]]
Willow warblers prefer young, open, scrubby woodland with small trees, including human-altered habitats such as coppice and young plantations up to 10–20 years old. High amounts of birch, alder and willow, with good lichen amounts, and water features (e.g. streams), fields with large amounts of bracken and mosses, and patches of low bramble (for nest cover) are preferred, but it will use a wide range of other species, including young or open coniferous forests. Incorporating woodland ride edge thickets of varying structure and height is beneficial. They prefer damp woodland areas. Thicket forming shrubs like blackthorn provide pockets of habitat. Deer browsing can degrade the required low cover.
The highest population densities are found in Scandinavia (where it is the commonest bird of any), with up to 1,100 pairs per square kilometre, and a total population in Sweden and Finland of 24 million pairs. Lower densities occur further east, with peak densities of 27 pairs per square kilometre in central Siberia. Even lower densities are found on the southern edge of the breeding range, with just 9 pairs per square kilometre in Switzerland, and a total of just 100 pairs in the whole of northern Spain.