Willow Warbler

The willow warbler (Phylloscopus trochilus) is a common species of leaf warbler found across Eurasia when breeding and Africa when non-breeding. It measures 11–12.5 cm (4.3-4.9 in) in length and weighs 7–15 g (0.25-0.53 oz). Its plumage is greenish-brown above and greyish-white to yellowish below, with plain greenish-brown wings that lack wingbars. Juveniles show a yellower underside compared to adults.

This species is very similar to the common chiffchaff (Phylloscopus collybita) but can be distinguished by its paler pinkish-yellow legs (as opposed to the dark brown to blackish legs of the chiffchaff), more elegant shape, and longer primary projection. Although plumage varies a lot in both species, the willow warbler tends to often have a bit more yellow in its plumage (this is more a tendency than a real distinction). The best way to tell the two species apart is on their songs, which are very different. The willow warbler has a melodious song, with soft tones in a repetitive descending whistle, whereas the chiffchaff’s song is less melodious, going “chiff-chaff-chaff-chiff-chaff-chaff” in the rhythm of a ticking clock.

The willow warbler thrives in a variety of habitats, from open woodlands to forest edges and even in gardens and parks in rural areas. These birds favor areas with high amounts of birch, alder, and willow, rich in lichens and mosses, and often near water features. It shows a preference for areas with dense undergrowth, where it can forage for its diet mainly composed of insects and other invertebrates. Willow warblers often employ a “hovering” technique when they pick off prey from leaves and branches.

The willow warbler is a long-distance migrant, breeding in Eurasia and wintering in sub-Saharan Africa. It undertakes a perilous journey twice a year, navigating thousands of kilometers to reach its breeding grounds in Europe and Asia. Some individuals travel up to 12,000 km between eastern Siberia to southern Africa. Willow warblers winter in sub-Saharan Africa from October to March, and migrate back to the breeding grounds from mid-March to mid-May.

The breeding season for the willow warbler starts in late April to May and ends in August. The bird constructs a dome-shaped nest low in dense foliage or on the ground, concealed among vegetation. Females lay 4 to 8 eggs, which are incubated for about 13 to 15 days. The chicks are altricial, requiring an additional 12 to 14 days in the nest before they are ready to fledge. Males are known to be territorial and may breed with more than one female, maintaining primary and secondary breeding territories.

The willow warbler is widespread and among the most common species of bird across most of its breeding range. There are over 34 million breeding pairs of willow warbler in Europe alone and an estimated 10 to 100 million further pairs across Russia. However, numbers seem to be decreasing, possibly due to habitat loss and changes in climate. Continued monitoring of populations is essential. It is listed as least concern on the IUCN Red List.