The Greenish Warbler Phylloscopus trochiloides is an abundant insectivorous Palearctic migrant breeding from eastern Europe across a broad swathe of the Boreal zone as far east as the Chinese coast. Birds winter in a far narrower area of the tropical deciduous forests of India, Sri Lanka, and east Asia. A small, slender, insectivorous species found in most wooded habitat, including parks and gardens, it forages actively and restlessly in the canopy, but will on occasion drop down to lower levels.

Greenish Warblers occur outside their normal range on a fairly regular basis, especially when weather conditions shift them off course - memorably about 35 turned up in the UK after fresh ENE winds in August 2007. Most birders in Europe and Asia consequently probably feel fairly confident about identifying them, and typical (or fresh) birds like the one so beautifully photographed by Hong Kong’s Paul J Leader and reproduced below (with his permission) show all the standard features that have long been used to separate Greenish Warblers from other similar phlloscopus warblers:

Greenish Warbler Phylloscopus trochiloides trochiloides, June 18 2005, Lake Khovsgol, Mongolia
Photo copyright Paul J Leader

• small, with a ‘large-headed’ look
• mainly olive grey-green upperparts with a yellowish-white long supercilium, which tends to broaden behind the eye and reaches the forehead
• clean, whitish underparts (note pale flanks) with a slight yellowish-green hue on the throat and upper breast
• generally plain wings with - importantly - a single whitish wing bar, which is rather neat and fine and formed by pale tips to the greater coverts
• no crown stripe
• dark upper mandible with a pale lower mandible
• greyish brown legs

Additionally the distinctive call is a loud, far-carrying, rather slurred disyllabic tchii-slii which is given throughout the year.

Fresh birds are not too difficut to identify, but a worn and tatty Greenish Warbler I photographed in Bangalore’s Nandi Hills this April was an altogether drabber bird (and is - I would guess - exactly what North American birders think most warblers this side of the world look like - ie bloody hard to identify!).

My own feeling is that if this bird was found in the UK in the spring even a more-experienced birder would find it puzzling. Where, they might justifiably ask, is the wingbar? Where - indeed - is the ‘greenish’ colour? Worn away, that’s where. Paul commented that studying the images I took does reveal a wing-bar, but some worn birds - and in the field I actually thought this one - may not show a wing bar at all. Paul additionally commented that my bird “is also moulting the tail [which is] not uncommon in phylloscs prior to migration and is showing signs of body moult on the head and flanks, this bird would certainly look much brighter is a week or two!”

What this bird demonstrates so well is that relying on a single feature - eg a wing-bar - or on an ‘overall’ plumage tone to identify an individual will sometimes throw a birder off course.

Looking again at the photos the bird shows MOST of the suite of characters that can be used to identify a Greenish Warbler: despite the wing-bar apparently lacking and the facial markings being indistinct the shape is correct, there is no crown-stripe, the bill has a dark upper mandible and a pinkish lower, and the legs are greyish-brown.

Many of the questions we’ve been getting sent into the 10,000 Birds clinic involve identifying birds that their finders say look like such-and-such “but aren’t quite right”. Worn or unusual plumage, feather loss, or even staining can change a bird’s appearance but concentrating on the features that look “right” rather than on the one or two features that look “wrong” is a far better way of correctly naming a bird.

And never forget the old phrase “If it looks like a duck, walks like a duck, and sounds like a duck - it’s a duck”: identifying my moulting Greenish Warbler was made easier because it was by far the most likely phylloscopus species to be in Nandi Hills in April, but even more helpful was the fact that even moulting, scruffy Greenish Warblers like this one call like every other Greenish Warbler on the planet. In this case if it looked (mostly) like a Greenish, behaved like a Greenish, and it sounded like a Greenish - it just had to be a Greenish…

(Thanks again to Paul J Leader for his helpful comments and image usage permission)