Willow Warbler

The willow warbler (Phylloscopus trochilus) is a small, long-winged leaf warbler breeding across a vast belt of northern Eurasia and wintering in sub-Saharan Africa. Adults are about 11–12.5 cm long and usually weigh 7–15 g. The plumage is softly greenish-brown above and whitish to yellowish below, with a pale eyebrow, fine bill and no obvious wingbars.

It is very similar to the common chiffchaff (Phylloscopus collybita). Willow warblers tend to look slightly longer-winged and more delicate, often with paler legs and a little more yellow in the plumage, but none of these features is completely reliable on its own. Voice is the easiest distinction: the willow warbler gives a flowing series of sweet notes that falls gently in pitch, very different from the repetitive song of the chiffchaff.

Willow warblers favour open woodland, young forest, scrub and woodland edges with a well-developed shrub layer. Birch, willow and alder are often prominent, particularly in northern Europe, but the species uses a wide range of wooded and semi-open habitats from lowlands to mountain birch forest.

Dense, mature closed-canopy forest is generally less suitable than younger or more open growth. The species benefits from a rich lower vegetation layer where it can forage through foliage while still having trees and shrubs available for song posts and cover.

The diet consists mainly of insects, spiders and other small invertebrates picked from leaves and fine branches. Willow warblers move almost constantly through the vegetation, gleaning prey from foliage and sometimes hovering briefly to take insects from leaf tips or undersides.

Berries and other fruit become more important late in summer and during migration. The birds therefore shift from a strongly insect-based breeding diet toward a somewhat broader diet as they prepare for and undertake their long journey south.

The male’s flowing, descending song is one of the most characteristic sounds of northern European woodland in spring and early summer. Males begin singing soon after arrival and may establish territories before nesting itself has begun.

The song is used to defend territories and attract mates and is repeated frequently from trees and shrubs across the territory.

The willow warbler is a true long-distance migrant: the entire breeding population leaves Eurasia and winters south of the Sahara. A major migratory divide crosses central Scandinavia. Birds from the southwestern part of the breeding range generally travel southwest through Iberia and the western Sahara to West Africa, whereas birds from northern and eastern Scandinavia tend to follow a southeastern route through the Balkans toward East and southern Africa.

Tracking published in 2018 showed the extreme scale of the eastern migration: birds of the subspecies P. t. yakutensis breeding in Chukotka in eastern Russia covered more than 13,000 km to wintering areas in eastern and southern Africa.

Orientation experiments published in 2012 found western, eastern and intermediate migratory directions even within the narrow contact zone in central Sweden. A tracking study published in 2023 also found that western-route birds were more likely to migrate during daylight while crossing major barriers, possibly reflecting the particularly demanding conditions of the western Sahara. The east–west difference is therefore a stable feature of the migration system rather than a simple response to one season’s weather.

The contrasting routes mean that neighbouring breeding populations can spend the non-breeding season thousands of kilometres apart. Conditions in separate African wintering regions can therefore contribute to very different population trends within a species that appears almost continuous across northern Europe in summer.

The female builds a domed nest with a side entrance, usually on or very close to the ground beneath grass, bracken, heather or other low vegetation. The outer structure is made from dry plant material and the inner cup is lined with finer fibres and feathers.

A typical clutch contains 5–7 eggs. The female performs the incubation for about 12–14 days, and the young normally remain in the nest for roughly 13–16 days after hatching. Both parents can contribute food to the chicks.

Most males pair with a single female, but some are polygynous and may attract a second female to another part of the territory. One brood is usual, although a second attempt can occur in favourable conditions.

The willow warbler is classified as least concern on the IUCN Red List. It remains one of the most numerous migratory songbirds across northern Eurasia and has an enormous breeding and wintering range.

Populations have declined substantially in parts of western Europe, while several northern populations have performed better. Because different breeding populations use different migration routes and African wintering areas, conditions at any stage of the annual cycle can contribute to these contrasting trends.