Birding in northern Thailand offers you a great opportunity to test your identification skill and increase your personal list of Phylloscopus warblers. The Blyth’s & White-tailed Leaf Warblers Complex is one of the phylloscopus groups that you’d most likely encounter when birding in montane evergreen forests like at Doi Inthanon, Doi Angkhang or Doi Lang. However, identifying them can be problematic at times, especially when you’re not familiar with any of them. This post will be the first part of the Blyth’s/Claudia’s/Davison’s Leaf Warbler Identification trilogy that I’m planning to accomplish. Let’s first explore here the general description of the plumage of each species.
To our current knowledge, 2 species from the Blyth’s Complex and 1 species from the White-tailed Complex can be found in northern Thailand; i.e. Blyth’s Leaf Warbler (P. reguloides assamensis) and Claudia’s Leaf Warbler (P. claudiae) from the Blyth’s Complex and Davison’s Leaf Warbler (P. davisoni) from the White-tailed Complex. Another species from the White-tailed Complex, Kloss’s Leaf Warbler (P. ogilviegranti intensior) is also found in Thailand but only restricted to the south-eastern part of the country.
Generally speaking, Blyth’s and Claudia’s cannot be separated solely by the plumage. The above photo is a Blyth’s Leaf Warbler (P. r. assamensis) from Doi Inthanon summit, the only place where I’ve seen this species in Thailand.
Characteristics of both Blyth’s and Claudia’s plumage are pale greyish median crown stripe contrasting to darkish lateral crown stripes, dark eye stripe, pale yellowish supercilium, no tertial fringes, no rump patch and darkish legs.
Typical appearance of P. r. assamensis, quite prominent greyish median crown stripe. Some yellow but not too much on the supercilium and face.
Median crown stripe is paler towards the back while lateral crown stripes get darker. Also note no tertial fringes nor pale rump patch unlike the Lemon-rumped Complex.
Claudia’s Leaf Warbler share pretty much the same tone of head pattern, body plumage and bare parts.
Davison’s Leaf Warbler, on the other hand, has more yellow tone to the plumage, especially at the supercilium, face and median crown stripe.
This photo clearly shows the yellowish tinge on the median crown stripe and the much yellower supercilium of Davison’s compared to Blyth’s and Claudia’s.
Structurally, Davison’s is also smaller, more delicate-built with probably weaker bill.
Blyth’s and Claudia’s often exhibit a strikingly large-billed appearance when seen in profile. This one is a Blyth’s Leaf Warbler from Doi Inthanon summit as well.
But of course, the impression of the bill strongly depends on the angle! This one is also a Blyth’s LW from Doi Inthanon summit.
The best feature to tell Blyth’s and Claudia’s from Davison’s is the undertail pattern. This is the typical undertail pattern for Blyth’s Leaf Warbler taken by Thanee Wongniwatkajorn at Doi Inthanon.
And here’s the typical undertail pattern of Davison’s Leaf Warbler taken at Doi Lang. Notice how the outermost pair of the tail feathers are almost completely white. While in Blyth’s and Claudia’s, they are mostly grey with thin white line on the outer web.
Actually, the amount of white on the outer web of outer tail feathers also differ between Blyth’s and Claudia’s (Blyth’s having slightly more white than Claudia’s) as you can see from the picture above (Blyth’s on the left and Claudia’s on the right), but the difference is too subtle to be used in the field. Other features like feeding behaviour, which will be for the next post, is much more useful for the identification of the two.
Now it may already sound easy to identify Blyth’s/Claudia’s from Davison’s, but look at this photo, the outer tail feathers seem to be completely white… so Davison’s?
The answer is NO. It’s actually a Blyth’s Leaf Warbler!
Because the outer tail feathers of these birds are so thin that they are translucent, sometimes they can appear totally white when spread against the light like in the photo above. So the best way is not to be so quick at identification and try to take as many photos as possible, especially when the undertail pattern is shown!
Even though the bill looks pretty large in this photo, the amount of yellowness and particularly the undertail pattern confirm that this is a Davison’s Leaf Warbler. Photo taken at Doi Lang.
But some Davison’s can be less strikingly yellow than the others. This individual was photographed at Doi Angkhang.
While some Blyth’s can show pretty striking yellow supercilium too, so never forget to check the undertail!
In the next post, I will write about behaviours of each species and how helpful it is to know the differences because it makes identification much easier than when looking at the plumage alone.