Willow warbler

The willow warbler (Phylloscopus trochilus) is, as its name suggests, a member of the warbler family, being similar in appearance to the chiffchaff but with brighter colouring. Although it is the commonest UK warbler it is not resident, being a summer migrant from southern Africa. It is found throughout most of Europe and as far away as northern Siberia.

Appearance

The willow warbler is about 11 centimetres long (a little over 4 inches). It has olive-green to brown upperparts and yellowish underparts, although the colouring changes around June as the back becomes browner and the chest whiter. Like all warblers, the willow warbler has a pronounced marking above the eye from front to back, known as a supercillium, the line being pale yellow in this case. Males and females have similar plumage.

Behaviour

The willow warbler can be distinguished from the chiffchaff by its song, which consists of a series of repeated notes getting lower in pitch but louder and then softer.

The willow warbler is not usually seen in gardens, being a woodland bird that prefers bushes, scrub and low trees that provide adequate cover. As the name suggests, willow copses are an ideal habitat.

The willow warbler flits from branch to branch, constantly flicking its wings and tail. An insect feeder, it is able to hover for short times as it takes insects and spiders from trees or the air.

Willow warblers generally leave for their winter quarters in September or October.

Breeding

Like the chiffchaff, the willow warbler nests on the ground or on a clump of heather or blueberry. The nest is a domed enclosed structure built from leaves, grass, moss and lichens and lined with soft materials. It is entered via a small side opening so that the eggs are not visible. The nest is built by the female who also does nearly all the incubating of the eggs. However, both birds help to raise the chicks.

The clutch, laid between May and July, consists of six or seven eggs that take around 13 days to hatch. The young leave the nest after about 15 days, although they stay close to the nest for another two weeks during which time the parents continue to bring them food. A second brood may be raised.

Despite the precautions taken by the female willow warbler in disguising the nest, this is a species that often falls victim to cuckoos, which insert their eggs in the nests of other birds. Willow warblers prove to be excellent but unwitting foster parents, continuing to raise the cuckoo chick long after their own nestlings (should they survive) have left.

Conservation

Despite their apparently healthy numbers and widespread distribution in the UK, populations of willow warblers have declined in recent years, which is why the species is on the “amber” list of birds whose survival is giving some cause for concern.

© John Welford