When talking about Little Brown Jobs (those small brownish birds that are difficult to identify), most birders in Thailand would immediately think of Phylloscopus warblers simply because there are so many of them. But for me, I’d say there are birds that are even more frightening to identify, the Acrocephalus warblers!
There are 8 species of Acrocephalus warblers, generally called “reed warblers”, in Thailand. They can be divided into 2 main groups, the large and the small sized 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 seems on the guide books!).
I find the most reliable characteristics which are useful for field identification between Oriental and Clamorous Reed Warbler include bill shape and the length of primary projections. Clamorous often strikes as being a large reed warbler with very slender and pointed bill, whereas Oriental has a noticeably thicker and stronger bill. Primary projections are also shorter and less pointed in Clamorous than in Oriental. Most Clamorous Reed Warblers also lack the whitish tips to tail feathers even in fresh plumage, but some odd birds can have this characteristic similar to Oriental, so it’s probably not as reliable as the former two characteristics.
So what about the other 6 species of reed warblers in Thailand? They are all small-sized, approximately about the same size as a sparrow, or just a little larger than most Phylloscopus warblers. The commonest and most widespread species among these small-sized reed warblers is the Black-browed Reed Warbler.
With a prominent black eyebrow over the pale supercilium, you might think that it’s always straightforward to identify this species. Most of the time, it is indeed, but every once in a while, you come across odd individuals that look like it might not be a Black-browed Reed Warbler.
The species that is mostly confused with Black-browed Reed Warbler is the globally threatened Manchurian Reed Warbler (or White-browed Reed Warbler by BirdLife). It is a range-restricted breeder in China that winters in South-East Asia. In Thailand, it is known to be a passage migrant in northern and central parts of the country and spend the winter along the eastern coast of southern Thailand. Recently, it was found to be wintering in Malaysia and Cambodia as well.
When compared to Black-browed, the Manchurian Reed Warbler generally has a different structure by having longer tail and longer bill. The lower mandible also lacks dark tip during winter unlike in Black-browed. The dark bill tip will only develop in spring and summer. Overall plumage is also warmer and more rufous-tinged similar to Paddyfield Warbler, which was once treated as conspecific. The black eyebrow is often faint and thin with the pale supercilium being narrower at rear unlike in Black-browed Reed Warbler which gets broader behind the eye.
Note: Beware that some Black-browed Reed Warblers can also show faint/thin black eyebrows too. More photos and identification notes on Manchurian/Black-browed Reed Warblers can be read from Dave Bakewell’s post here.
Things get even more confusing with the small-sized reed warblers that lack black eyebrows. Let’s start with the Paddyfield Warbler, a bird that is widespread across Europe and Asia but is a rare winter visitor to Thailand. Once it was treated as the same species as the Manchurian Reed Warbler and it was no surprise. They actually share many similarities like warm rufescent-brown plumage and very similar wing formula and length of primary projections. Most Paddyfield Warblers show hints of dark eyebrow over the pale supercilium too, but never as pronounced as in Manchurian/Black-browed Reed Warblers. However, unlike in Manchurian Reed Warbler, the Paddyfield Warbler always shows dark tip to lower mandible throughout winter. The bill is also shorter and more pointy at tip than in Manchurian. In Thailand, it can only be found at several locations in northern part of the country including Nam Kham Nature Reserve in Chiang Rai where these photos were taken.
The most challenging group of all reed warblers, and probably all Little Brown Jobs in Thailand, is the small-sized Acrocephalus warbler without dark eyebrows. There are 3 species in this group for 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, longish bill and rather weak supercilium making them look quite plain-faced.
The commonest and most widespread species among the three is the Blunt-winged Warbler. It winters in wetlands throughout northern and central Thailand. Despite its abundance, it’s never easy to find one because of its extremely secretive behaviour. Even if you finally manage to see one, the bigger question is how to identify it! It’s almost impossible to identify these small-sized and plain-faced reed warblers without seeing the wing formulae.
The Blyth’s Reed Warbler is a rare winter visitor that has only been recorded several times at 2 different 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 generally has colder and more greyish-tinged plumage with much longer primary projections and slightly shorter tail. Its P1 is also shorter and doesn’t markedly extend beyond primary coverts unlike in Blunt-winged Warbler. It also usually prefers dryer habitats like shrubs and forest edge near wetlands, while Blunt-winged Warbler is a strictly wetland species that can be found only in reed beds.
Finally, the most challenging identification among all reed warblers is between the Blyth’s and Large-billed Reed Warbler. The Large-billed Reed Warbler is an extremely rare and 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. It has also been recorded several times at Bung Boraphet afterwards. Generally, it has longer bill and warmer tone to the plumage than in Blyth’s Reed Warbler, but these two species can overlap in almost every morphological aspect. There was even a bird caught at Nam Kham Nature Reserve that couldn’t be identified even in hand, and DNA test was required. In the end, it turned out to be a Blyth’s Reed Warbler. Because of all the difficulties, I’m not going to touch on how to identify these two in the field and I’ll leave this post with a photo of a Large-billed Reed Warbler from Nam Kham Nature Reserve instead.