When talking about Little Brown Jobs or those small birds that are difficult to identify, most birders in Thailand would immediately think of the Phylloscopus warblers because there are so many species of them. They’re definitely one of the birders’ ultimate nightmares. But for me, I’d say there are something even more frightening than identify the Phyllocs, the Acrocephalus warblers!
There are 8 species of Acrocephalus warblers, or generally called “reed warblers”, in Thailand. They can be divided into 2 main groups, the large and the small reed warblers. Only 2 species are considered as large reed warblers; the Oriental Reed Warbler and the Clamorous Reed Warbler. These two are approximately 20 cm in length or about the size of a bulbul. The latter is an extremely rare vagrant OR a highly overlooked species in Thailand with very few accepted records, while the former is the commonest and most widespread reed warbler in the country. The identification of these two species is extremely challenging (definitely more challenging than what it seems in the guide books!). I still haven’t been able to scrutinize all the large reed warblers that I’ve seen, so I assume that all of them are Oriental Reed Warbler for the meantime.
What about the other 6 species of reed warblers in Thailand? They’re all small-sized reed warblers which are approximately about the same size as a sparrow, or just a little larger than most Phyllocs. The commonest and most widespread species among these small reed warblers is the Black-browed Reed Warbler.
With a prominent black eyebrow over the pale supercilium, you might think that it’s quite straightforward to identify this species. Most of the time, it is indeed, but every once in a while, you’ll come across odd individuals that look like it might be a different species.
The species that is most often confused with Black-browed RW is the globally threatened Manchurian Reed Warbler. It is a range-restricted breeder in China that winters in South-East Asia. In Thailand, it is reported to be a passage migrant in the north and central part of the country and spend winter along the eastern coast of southern Thailand. Recently, it was also found to be wintering in Malaysia and Cambodia too.
When compared to Black-browed RW, the Manchurian RW generally shows a different structure by having longer tail and longer bill. The lower mandible also lacks dark tip unlike in Black-browed RW which shows dark tip to lower mandible throughout winter. In Manchurian RW, the dark tip is only developed in spring and summer. The plumage is also warmer and more rufous-tinged similar to Paddyfield Warbler, which it was once treated as the same species. The black eyebrow is often faint and thin and the pale supercilium tend to be thinner and more pointy at rear end than in Black-browed RW.
Note: be aware that some Black-browed RW can also show faint/thin black eyebrow too! Better photos and more identification notes on Manchurian/Black-browed RW can be read from Dave Bakewell’s post here.
Things get even more confusing with the small reed warblers that lack black eyebrow. Let’s start with the Paddyfield Warbler, a bird that is quite widespread in Europe and Asia but is a rare winter visitor in Thailand. The Manchurian Reed Warbler was once treated as a subspecies of Paddyfield Warbler and it’s no surprise. They share many similar characteristics like warm rufescent-brown plumage and very similar wing formula and length of primary projection. The Paddyfield Warbler can often show a hint of dark eyebrow over the pale supercilium as well, but never as black as in Manchurian/Black-browed RW. However, unlike in Manchurian RW, the Paddyfield Warbler always shows dark tip to lower mandible throughout the winter. The bill is also shorter and more pointy at tip than in Manchurian RW. In Thailand, it can only be found in several locations in northern part of the country including Nam Kham Nature Reserve in Chiang Rai where these photos were taken.
But the most challenging group of all reed warblers, or probably all LBJs, is the plain-faced small Acros! There are 3 species recorded in Thailand; the Blunt-winged Warbler, Blyth’s Reed Warbler and Large-billed Reed Warbler. All three species share some very similar characteristics including the lack of dark eyebrow, long bill and shortish supercilium which is not so prominent making them look rather plain-faced.
The commonest and most widespread species among the 3 is the Blunt-winged Warbler which is a winter visitor to wetlands throughout northern and central Thailand. Despite its abundance, it’s never easy to see one because it’s a very shy and skittish bird. Even though you finally get to see it, the bigger challenge would be to identify it!
It’s almost impossible to identify these small plain-faced reed warblers without seeing the wing formula.
The Blyth’s Reed Warbler is a rare winter visitor that has only been recorded several times from 2 locations in Thailand; i.e. Nam Kham Nature Reserve in Chiang Rai and Bung Boraphet in Nakhon Sawan. Compared to the Blunt-winged Warbler, it has colder and overall more greyish-brown tone to the plumage with much longer primary projection and shorter tail. Its P1 is also shorter and doesn’t markedly extend beyond primary coverts like in Blunt-winged Warbler. Behaviour-wise, the Blyth’s RW prefers dryer areas with scrubs or forest edge near wetlands, while Blunt-winged Warbler is a strictly wetland bird that can only be found in reed beds.
Finally, the most challenging identification among all reed warblers (and LBJs, in my opinion) is between the Blyth’s and Large-billed RW. The Large-billed Reed Warbler is an extremely rare and very little known species which has only been recently rediscovered in 2006 by Philip Round at Laem Phak Bia in Phetchaburi and followed by another record from Nam Kham Nature Reserve in 2008. Basically, the Large-billed RW has longer bill and warmer tone to the plumage, but these two can overlap in almost every morphological aspect. There was even one bird at Nam Kham Nature Reserve that we weren’t able to identify even after measuring it in hand, so DNA test was required! In the end, the DNA result revealed that it was a Blyth’s Reed Warbler. I’m not going to touch on how to identify these two, so here’s a photo of a true Large-billed Reed Warbler from Nam Kham Nature Reserve instead.