The Willow Warbler is a small, greenish warbler which is very common right the way across Europe in the summer months, arriving in late March or early April and departing in September and October. Unlike those of its close relative the Chiffchaff, Willow Warbler wintering records, in Britain at least, are still exceptional.
Willow Warblers are found in the same habitats as the Chiffchaff, but tend to favour smaller, younger trees, bushes and ground vegetation. They can also be seen in quiet woodland-fringe gardens, or in those which are somewhat overgrown.
A small bird, the Willow Warbler bears a very strong resemblance to the stockier Chiffchaff.
Check the structural points – look at the long wing length of the Willow Warbler (short on Chiffchaff) – the leg colour (pale on Willow Warbler, dark on Chiffchaff) and overall plumage tones. If all else fails, listen for the song, which is a series of fluid descending notes, ending with a rapid flourish. The call note is similar to that of the Chiffchaff, but is a more penetrating and musical ‘hoo-eet’.
Willow Warblers in spring are generally paler in appearance than Chiffchaffs. The head and upperparts are pale olive-green, with a yellowtinged long supercilium, not short and buff as on the Chiffchaff. A pencil-thin black eyestripe and slightly blotched cheeks are also apparent. The wings are pale olive, except for darker primaries. The tail is also dark. The underparts are cleaner than the Chiffchaff’s, lacking any buff tones. The breast and flanks are washed pale yellow and the belly is whitish.
The bill shows a dark upper mandible and tip, with a flesh-pink lower mandible. The eye is black, with a less obvious ring than the Chiffchaff and the legs are pale flesh.
Occasionally, in Britain, you may encounter a very pale-looking ‘northern’ Willow Warbler. The whole of the head and upperparts are a washedout pale greenish colour, except for darker primaries, tail tip and eyestripe. The supercilium and underparts are off-white, with a greyish suffusion on the flanks. These striking birds may invite confusion with rarer warblers, so beware!
Basically resembling a spring bird, the adult Willow Warbler in autumn may look a little browner on the upperparts and more yellowy below.
Some young Willow Warblers in autumn can be particularly striking. The upperparts are pale olivegreen, while the underparts are rich lemon, from the face and throat to the undertail-coverts, although the belly usually stays whitish. The bare parts are much as the adult’s.