Birding in northern Thailand offers you a great opportunity to test your identification skills and increase your personal list of Phylloscopus warblers. The Blyth’s & White-tailed Leaf Warbler Complex is one of the Phylloscopus groups that you will most likely encounter while birding in montane evergreen forests like at Doi Inthanon, Doi Ang Khang or Doi Lang. However, identifying them can be challenging at times, especially when you’re not familiar with any of them. This post will try to sum up the identification of this Phylloscopus group that can be regularly seen in northern Thailand.
To our current knowledge, 5 species from this complex can be found in Thailand, i.e. Blyth’s Leaf Warbler (P. reguloides), Claudia’s Leaf Warbler (P. claudiae), Hartert’s Leaf Warbler (P. goodsoni), White-tailed Leaf Warbler (P. intensior), and Kloss’s Leaf Warbler (P. ogilviegranti).
Blyth’s Leaf Warbler is a resident bird of montane evergreen forest in northern Thailand. Currently, it is known from Doi Inthanon, Doi Chiang Dao, Doi Pha Hom Pok and Doi Lang, all in Chiang Mai province. Claudia’s Leaf Warbler is a widespread winter visitor throughout most of Thailand, except in the south. It can be found in various habitat types including montane and dry evergreen forest, mixed-deciduous forest, and even in urban parks during passage. White-tailed Leaf Warbler is also a resident bird of montane evergreen forest similar to Blyth’s Leaf Warbler, but it can also be found in the west and north-east (race muleyitensis), and also in the south-east (race intensior).
Hartert’s Leaf Warbler is a little known migrant with only one record of the nominate race from Bangkok, while the identification and distribution of the race fokiensis is still very little understood. As for Kloss’s Leaf Warbler, there is a historical record of the subspecies disturbans, but that’s pretty much the only thing we know about it at the moment.
This post will only cover the identification of 3 species including the Blyth’s, Claudia’s and White-tailed Leaf Warbler as they are the most frequently seen members of this complex.
Characteristics of all 3 species include prominent wing bar on both greater and median coverts, strong eye stripe, pale median crown stripe and yellow lower mandible with no black tip. In Claudia’s Leaf Warbler, the head and underparts are rather cold greyish with a slight yellowish wash on the face and supercilium. Blyth’s Leaf Warbler is slightly more yellowish overall, and White-tailed Leaf Warbler usually appears the most yellowish, particularly on the underparts, supercilium and facial area. Structurally, White-tailed Leaf Warbler also appears to have a slightly weaker bill than the other two species.
But the most diagnostic feature of the 3 species is the undertail pattern. In Blyth’s and Claudia’s Leaf Warbler, the undertail is pale grey with white edge to the inner web of the outermost tail feathers. In Blyth’s Leaf Warbler, the white edge is bolder than in Claudia’s Leaf Warbler, especially at the tail tip. White-tailed Leaf Warbler has a very different undertail pattern. The race muleyitensis of N, W, and NE Thailand has all white inner web of the outermost tail feathers, while the race intensior in the SE has about half less white on the outermost tail feathers.
It is also important to pay attention to the behaviour of the birds that you’re observing. Blyth’s and White-tailed Leaf Warbler are pretty much normal Phylloscopus warblers, i.e. feeding by hopping from twig to twig looking for small insects among and under the leaves. The White-tailed Leaf Warbler might tend to hover and snatch food items from midair more often than Blyth’s Leaf Warbler.
Claudia’s Leaf Warbler, on the other hand, has a very unique way of foraging. It almost always clings on tree trunks and branches similar to a nuthatch. The behaviour is strikingly different from other similar species (except Hartert’s Leaf Warbler, which also exhibits the nuthatch-like behaviour, but we won’t go there!). Because of this, Claudia’s Leaf Warbler usually appears long-necked and slender-built because it always try to look out for food hidden under the tree trunks and branches while climbing.
But be careful, Blyth’s Leaf Warbler can also sometimes perch on vertical branches and hang upside-down to look for food, but it would normally skip on to the next branch shortly, not climbing along the same branch like Claudia’s Leaf Warbler mostly do.
Apart from the differences in plumage and feeding behaviour, the song of each species is also very useful for the identification. The songs of Blyth’s and White-tailed Leaf Warbler are quite similar and would required a trained ear to separate them in the field. However, the song of White-tailed Leaf Warbler is slightly more rushed and slurred.
Blyth’s Leaf Warbler‘s song
White-tailed Leaf Warbler‘s song
The song of Claudia’s Leaf Warbler, however, sounds very much different. Unlike the melodious song of both Blyth’s and White-tailed Leaf Warbler, the song of Claudia’s Leaf Warbler mainly consists of a series of repetitive notes, so the identification is quite straightforward.
Claudia’s Leaf Warbler‘s song
I hope you will find this post useful when getting to see them in real life. The problem with observing them in the field is that you don’t usually get to have many good views of the same bird. It requires some extra effort in following the same bird long enough to get a good look at it, or a good photo that can be used for the identification. For me, that just makes birding a little more fun!