The problem

Early field guides such as the ubiquitous A Field Guide to the Birds of Britain and Europe (Peterson et al 1954) engendered the simplistic view that Willow Warbler and Common Chiffchaff could be distinguished by leg colour: the former had orange legs, the latter black. However, the variability of this characteristic was known by the 1960s, and the pervasive view then was that it was ‘unscientific’ to separate them in the field. Apart from their songs, the only way to tell them apart was to catch them and examine their wing formulae.

Most birders in the 1960s and 1970s duly gave up trying to identify them, recording them instead as ‘willow-chiffs’ or as Phylloscopus sp’. It took a new generation of energetic young birders in the late 1970s and 1980s to adopt a more positive attitude.

Despite this refound confidence, it has to be stressed that the two species are very similar. What is more, they are most similar in spring when they return from their winter  quarters: Common Chiffchaffs mainly from the Mediterranean, North and West Africa, and Willow Warblers from sub-Saharan West Africa (with more easterly breeders penetrating right down into South Africa).

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The solution

Primary projection

It is the fundamental difference in their migration strategies that provides us with the most reliable method of telling these two species apart. Being a long-range migrant, Willow Warbler has longer wings than Common Chiffchaff: 59-72 mm, compared with 54-67 mm (Svensson 1992).

Despite the overlap, the difference manifests itself in the length of the visible primaries relative to the length of the overlying tertials. Willow Warbler has long primaries that are approximately three quarters or equal to the overlying tertial length, whereas Common Chiffchaff’s short primaries are only about a third to a half the overlying tertial length. The visible primaries at rest are usually referred to as the ‘primary projection’ and, as a general rule, longer-range migrants have a longer primary projection than short distance migrants. Of course, evaluating the primary projection on a moving warbler may prove frustratingly difficult, particularly if it is overhead, so a good deal of patience and perseverance is essential.

The longer wings of Willow Warbler produce another consequence for its identification. It has a somewhat faster, more dashing, more direct and more purposeful flight, whereas the shorter winged Common Chiffchaff has a slightly more hesitant and somewhat tit-like action. Although such differences are very subtle, they are nevertheless real.

Plumage

In spring, the plumage of the two species is similar but Willow appears slightly more ‘washed out’ and greener above, and it tends to show subtle primrose tints to the face and throat. Common Chiffchaff tends to be slightly more olive above, with little yellow on the face and underparts. More significant is the fact that Willow has a better-defined, ‘sharper’ facial pattern, with a prominent supercilium and a stronger dark eye-stripe. Common Chiffchaff’s less well-defined face pattern may be dominated by a noticeable pale eye-ring but, although obvious on freshly moulted birds in autumn, this is usually less apparent in spring.

Leg colour

As the old field guides suggest, leg colour is a useful difference: Willow has orangey or pale brown legs, Common Chiffchaff blackish. The problem is that this feature is not 100 per cent reliable. Although I have no recollection of ever seeing a Common Chiffchaff with pale legs, some Willows may show dark legs, even if such birds usually retain orange feet. Remember also that leg colour varies according to the light.

Tail-dipping

A useful behavioural trait to watch out for is that Common Chiffchaff habitually dips its tail downwards when feeding. The exact purpose of this is unclear, but I suspect that it facilitates prey detection. Willow Warbler tends to keep its tail straight and still when feeding. It must be stressed, however, that Willow Warbler also tail-dips, sometimes quite persistently, particularly after landing, but habitual, deliberate and exaggerated downward tail-dipping is a sure sign of a Common Chiffchaff.

Differences in their tail movements emphasise another point: Willow is a sleeker, more attenuated bird than Common Chiffchaff, which is more rounded and less streamlined. Again, this may be a subtle difference, but keep looking at them and you will see what I mean.

Voice

Of course, the easiest way to separate them is by song. Common Chiffchaff’s monotonous, slow two-note chiff-chaff song can be heard right through the summer and into the autumn. Willow Warbler’s pleasing song is one of the classic sounds of spring: an understated soft, steady, gentle, descending lilt. Note that birds on passage give this as a rather quiet and subdued sub-song. Both species have a soft huu-eet call. Willow’s is stronger and more disyllabic, whereas Common Chiffchaff’s is flatter and more slurred, but such are the vagaries of delivery, wind direction, background noise and so on that I often find them difficult to separate on call alone.

Habitat preferences

Autumn birds

We have concentrated on spring birds, but it is worth remembering that in autumn the two species are much more easily separated. Juvenile and first-winter Willow Warblers are distinctly green above and quite a bright primrose yellow over the entire underparts, as are adults once they have completed their post-breeding moult. Late summer Common Chiffchaffs are invariably scruffy juveniles showing loose, fluffy body plumage with grey tones to the head, dull underparts and greenish upperparts, often with prominent yellow rim to the bend of the wing. Such birds can be seen well into September. Adults are similarly dull and scruffy during their prolonged post breeding moult, which may also extend into September. By this time most Willows will have already moulted into their pristine yellow-and-green winter plumage and headed off to Africa. Freshly moulted late autumn Common Chiffchaffs of the nominate race collybita, which breeds in Britain, are a rather dull olive-and-buff with a well-defined pale eye-ring.

Whereas Willow Warblers continue to give their normal, cheerful, up-slurred huu-eet call in autumn, Common Chiffchaffs in late summer and early autumn often give a more distinctive, quite loud, down-slurred wee-oo. Traditionally, it has been thought that this is given solely by juveniles but I recently saw and heard an adult in primary moult giving this call. Why they change their call at this time of year is unknown.