Green and Greenish Warblers have always been notoriously difficult to tell apart. Till recently, Green Warbler (P. nitidus) was considered to be a subspecies of the Greenish Warbler (P. trochiloides). Considering the difficulties involved in identifying the two species in the field, I hope the few pointers here are helpful. Do note that the species are best separated by a combination of call and plumage*.
The subspecies of Greenish Warbler in our region are the nominate trochiloides, ludlowi (not discussed here) and viridanus. P. viridanus is known to breed in the N and NW Himalayas as well as Europe and winters extensively in S India. While some individuals do spend the winter in N/NW India, they pass through that region in larger numbers on passage. The nominate subspecies (trochiloides) is known to breed in C and E Himalayas and winters mostly in E India although individuals are known to straggle all the way across to S India as well.
Note: In the coastal region in the states of Kerala, Karnataka and Goa, the Green Warbler is a lot more common than the Greenish Warbler.
*In our region, most migratory Phylloscopus sp are likely to show worn plumages in the latter half of winter (January to April), and so call is an important additional diagnostic.
Behaviour: Similar in both birds.
Quick, nervous movements with regular wing- and tail-flicking by both species. Also noted to hover and make sallies. While they may occur across all strata of vegetation, both prefer trees (of all sizes) as opposed to shrubs and bushes.
Bill: Similar for P. nitidus and P. t. viridanus (pale lower mandible in both birds). P. t. trochiloides has a darker bill (the lower mandible shows some pale at the base).
Range: Based on current knowledge, the Green Warbler seems to be more restricted to S, SW India and Sri Lanka than the Greenish Warbler – which is widespread across the subcontinent.
The following migration maps clearly illustrate the differences in the summering as well as wintering ranges of the two species: (Look at how some Greenish Warblers ssp viridanus travel all the way to India from central and eastern Europe!)
Plumage: Green is generally brighter, more marked than Greenish – but be wary of moult!
Note number and strength of wing bars*, face (including cheeks and supercilium), overall colouration.
*Note on wing bars: Here, the bar formed by the median coverts is referred to as the first (or upper) wing bar and the bar formed by the greater coverts is referred to as the second (or lower) wing bar.
In fresh plumage, Green Warbler shows a distinct yellow wash on the face (including supercilium) often running all the way down past the throat to the breast. It is visibly bright green above and usually shows two distinct white wing bars – with the second wing bar appearing thicker and more prominent.
In contrast, a Greenish Warbler in fresh plumage is duller green above, dull whitish below with some mottling and shows one wing bar.
In worn plumage, most Green Warblers shows the second wing bar, but it may appear broken, weak or missing in the most extreme cases. The upperparts are not as bright as fresh plumage and become “greyer” while the yellow is reduced in brightness and extent, though the supercilium and face retain a yellow wash unlike on Greenish Warbler. This plumage may resemble a viridanus ssp of Greenish Warbler in fresh plumage with the exception of the yellow on the face.
On a Greenish Warbler, the wing bars may wear off altogether – thus giving it a very plain and dull appearance. The viridanus ssp is especially uniform and plain overall while the nominate shows mottled, slightly darker underparts and contrasting flight feathers when wing is closed.
Call: With some practice, it is fairly easy to tell Green Warbler apart from Greenish Warbler by call.
Green Warblers have a trisyllabic call that can be transcribed as a “chi-su-wee”. The middle syllable may often be hard to catch by ear and the call is often delivered so fast that it may just sound like a slurred note. However, once familiar – it is hard to mistake the call with anything else. The call, as can be seen from the sonogram, ends with an upslur (the sound rises in pitch).
Greenish Warblers have a distinctly disyllabic call. The call of the viridanus ssp is more subtle, often sounding like a “chee-up” (not unlike a Brooks’s Leaf Warbler, for those familiar with the species). The first syllable is more pronounced than the second syllable, which is abrupt and ends with a downslur (sound falls in pitch). The call of the nominate ssp is an even more obvious disyllabic “chiwee” that is perhaps more reminiscent of a Hume’s Warbler. However, the call of the Hume’s Warbler is less slurred, louder and “sweeter” sounding than nominate Greenish.
Note: It is often worth waiting for a regular bout of calls to begin since both warblers do vocalize with slight variations to the “typical” call. When vocalizing again after a break from calling, Greenish Warblers (especially viridanus) often call with a single note. Green Warblers, too, occasionally chatter with a disyllabic call for a long time (especially when excited), thus sounding much like Greenish Warblers!