The Acrocephalus Issue

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!

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Oriental Reed Warbler is the commonest and most widespread Acros in Thailand.

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Mostly seen foraging along scrubs and grassy area near water.

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Frequents perching in the open to dry itself up in the morning. Note pale tip to tail feathers.

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Can be rather showy and perch on grass top for a look out or to sing before spring migration

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Note long and pointy primary projection

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Often shows hint of greyish streaking on breast (more prominent in spring)

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.

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Among the small reed warblers, the Black-browed Reed Warbler is the commonest species.

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It’s about the same size and at times can resemble a Phylloscopus warbler like Dusky Warbler or Radde’s Warbler.

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Most of the time, the black eyebrow is thick and obvious enough to separate it from other similar species.

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But other times, the black eyebrow can appear faint and thin.

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But that might depend on the angle!

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Or the angle plus lighting

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.

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Manchurian Reed Warbler is listed as Vulnerable by the IUCN Red List. I photographed this bird in my local patch in Mae Taeng, Chiang Mai on 18 September 2004. It is still by far the only record of this species in the north.

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Note completely pale tip to lower mandible (mostly blackish in Black-browed RW). For Manchurian RW, this can turn blackish only in spring and summer.

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Structurally, it is more similar to Paddyfield Warbler (which was once treated as the same species) than Black-browed RW. The tone of plumage is also warmer and more rufous-tinged like Paddyfield Warbler rather than Black-browed RW as well.

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.

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Paddyfield Warbler is a rare winter visitor to small areas of northern Thailand.

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This adult bird was ringed at Nam Kham Nature Reserve in February 2014 and was found to be a regular visitor at the Rubythroat Hide in 2016.

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Bill is rather short and shows prominent dark tip to lower mandible throughout winter.

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Plumage is warm rufescent-brown with rather short primary projection

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.

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Blunt-winged Warbler is one of the trickiest birds to see and even trickier to identify!

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It’s one the small reed warblers that lack any trace of dark eyebrow, which makes it look rather plain-faced.

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I probably wouldn’t be able to identify this bird without this lucky, even though blurry shot that shows the wing formula.

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Note the long P1 that extends beyond primary coverts

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Structurally, it’s a small Acros with long tail.

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Supercilium is short and not as prominent as in Paddyfield Warbler.

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.

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Left: Blyth’s Reed Warbler; Right: Blunt-winged Warbler

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Left: Blunt-winged Warbler; Right: Blyth’s Reed Warbler. Note the short and blunt primary projection as suggested by the name in Blunt-winged Warbler.

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The long P1 which extends beyond the primary coverts is another main ID feature for Blunt-winged Warbler.

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Blyth’s Reed Warbler is a rare winter visitor in Thailand. Note the much shorter P1 and shorter tail than in Blunt-winged Warbler.

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Primary projection is much longer and more pointed than in Blunt-winged Warbler.

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Blyth’s Reed Wabler (left) tends to have colder brown and often grey-tinged plumage than in Blunt-winged Warbler (right). The supercilium is also more obscured in Blyth’s RW.

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Blunt-winged Warbler in profile; note rather warm brownish plumage and short but distinct supercilium.

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Blyth’s Reed Warbler in profile; note cold greyish-brown plumage and less prominent supercilium

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When seen in the field, the Blyth’s Reed Warbler is generally a long-billed and short-tailed bird.

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.

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The second record of Large-billed Reed Warbler in Thailand from Nam Kham Nature Reserve. Photo by Woraphot Bunkhwamdi.

Nam Kham Nature Reserve: 28-29 March 2014

On the evening of March 28 saw me arriving at Nam Kham Nature Reserve in Chiang Saen, Chiang Rai Province with my friends who are also bird ringers. They had to prepare poles and nets for the following morning, so I went into one of the hides to see if there’s any bird at the pool made for birds to come and bathe. To my surprise, as I stepped into the hide, the rare Chestnut-crowned Bush Warbler was already bathing, as well as a male Siberian Rubythroat. They quickly flew into the bush as they noticed me but came out shortly afterwards.

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The Chestnut-crowned Bush Warbler came down to the pool 3 times until it got completely dark, but I could only get a few ok-ish shots. This is a rare and quite mysterious species that overwinters in northern Thailand. We only knew that it spends the winter in lowland reed beds just a few years back when Nam Kham was established. On the other hand, it moves up to high montane forest to breeding during breeding season.

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While the bush warbler was away, I was entertained by a family group of the colourful Chestnut-capped Babbler. Two parent birds and two juveniles came to enjoy evening bath for more than 10 minutes. Both of the parents were ringed just like the bush warbler and most birds here. Another bird which came to the pool was the lovely Baikal Bush Warbler, but it was already too dark for me to get any good shot.

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A ruin at the entrance from the main road to Nam Kham Nature Reserve

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Chestnut-capped Babbler looking really smart when seen up close

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Yellow-bellied Prinia was particularly brightly coloured.

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Male Siberian Rubythroat

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The throat somehow reminds me of an almost ripe strawberry.

On the following morning, I joined a ringing session where we caught some really nice birds. The first one that I saw was a smart adult Chestnut-capped Babbler followed by a very brightly coloured Yellow-bellied Prinia. A beautiful male Siberian Rubythroat was also caught as well.

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Blyth’s Reed Warbler, a Thai rarity and a lifer for me

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Long primary projection relative to the similar Blunt-winged Warbler

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Wing formula of Blyth’s Reed Warbler

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But the highlight of the session was a single Blyth’s Reed Warbler, one of the rare Acrocephalus warblers in Thailand. Few records have beeb reported from Nam Kham Nature Reserve and Bung Boraphet in Nakhon Sawan Province.

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One of the two Blunt-winged Warblers caught on that morning

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Blyth’s Reed Warbler (left) and Blunt-winged Warbler (right)

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Blunt-winged Warbler (left) has shorter primary projection than Blyth’s Reed Warbler (right).

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When compared closely, Blyth’s (left) shows paler and more rufous iris, while Blunt-winged (right) has a colder brown tone. However, iris colouration can be greatly varied.

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Focusing on the Blunt-winged Warbler

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Now focusing on the Blyth’s Reed Warbler

We also caught 2 Blunt-winged Warblers, a rather common but extremely difficult to see species, so we had a really nice opportunity to compare the two species closely together. The best way to distinguish the two is by looking at the primary projection. Blunt-winged Warbler has shorter primary projection than Blyth’s making the wings look shorter and rounder, hence the name Blunt-winged. Apart from the primary projection, I also noticed that the iris of the two species were slightly different; Blyth’s having a lighter and more rufous colouration than Blunt-winged which has a cold brown tone to it.

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The same Blyth’s Reed Warbler after being released

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At least the long primary projection is visible!

Since there had been no confirmed photo of Blyth’s Reed Warbler taken in Thailand in the wild, I prepared to take photos of it after being released. The above shots were already my best effort. Unfortunately some part of the bill is hidden in both shots but at least the long primary projection is still visible confirming that it is not a Blunt-winged nor Paddyfield Warbler.

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I then took a walk around the reed maze and came across 2 singing Acrocephalus warblers in tall reeds. Since one of them was so close, I tried recording the song using my iPhone and played back. Quite amazingly, despite the very low volume, the bird still responded and came out to perch on reed top for few seconds, enough for me to snap some shots before it flew out. One of the shots shows wing formula which seems to suggest that it’s a Blunt-winged Warbler (relatively long P1).

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Before leaving Nam Kham, I spent about an hour sitting in the hide waiting for birds to come and bathe. Only a Baikal Bush Warbler showed up briefly. I guess it was the same individual that showed up in the earlier evening. It was still in a non-breeding plumage, showing no sign of grey on breast nor dark lower mandible which are characteristics of breeding plumage.