Pycnonotidae is among the most diverse bird families in Thailand. There are 40-41 species recorded here (depending on the checklist that you use). Some members of the family have colourful plumage and cheerful songs, while many others are noisy, brown and boring. Members of the genus Iole belong to the latter. In Thailand, there are 5 species (or 4, or 3!) within this genus. They all look pretty much the same and have the same kind of funny nasal calls. Through this post, I’ll share some of the notes and photos that I’ve taken of each Iole bulbul throughout Thailand.
Let’s go through the members of this genus one by one. The first species is actually a rather new member to this genus, the Finsch’s Bulbul (Iole finschii). Originally, it was placed within either genus Criniger or Alophoixus, but the current version of most checklists including the IOC and HBW Checklist, place it within the genus Iole. Despite looking very different from other Iole bulbuls, it actually has a similar type of call to other members of the genus. The call of Finsch’s Bulbul can be listened here. Plumage-wise, it has a distinct yellow wash on the underparts with a distinctive yellow puffy throat different from other species. The bill is also thicker than other Iole bulbuls making it the easiest member of this genus to identify. In Thailand, it can only be found in the southernmost part of the country along the Thai-Malay border.
The second species, the Olive Bulbul (I. viridescens) also has a restricted distribution in Thailand. It can be found only in the western part of the country along the Thai-Myanmar border, mainly in Tak and Kanchanaburi province. The distribution of each species is one of the most important clues for the identification of Iole bulbuls in Thailand. To our current knowledge, there is hardly any overlap between the distribution of Olive Bulbul and other Iole bulbuls. This makes the identification quite straightforward when birding in the known range of Olive Bulbul, such as Mae Wong National Park, Thung Yai Naresuan Wildlife Sanctuary or Taksin Maharat National Park. Apart from the distribution, it also has a more yellowish-olive tone to the plumage compared to other species, hence the name. However, it can also be difficult to really judge the tone of the plumage depending on the light. It also has a different shade of iris colour which usually appears slightly darker and more reddish than in other Iole bulbuls (again, can be difficult to judge depending on the light). The call of Olive Bulbul can be listened here https://www.xeno-canto.org/336903.
The next species is the Baker’s Bulbul (I. cinnamomeoventris). It is a rather new split and a taxonomic headache. The Thai Bird Records Committee accepts it as a species according to a study on morphology, calls and genetics of all Iole bulbuls. Other checklists place it within different species. Most checklists treated it as a subspecies of Grey-eyed Bulbul, but the IOC and Clemenets recently placed it within Olive Bulbul. Despite the taxonomic complication, it is a rather distinctive taxon which deserves your attention. In Thailand, it can be found along the south-western border all the way to the western part of the Thai-Malay Peninsula. If you’re birding at Kaeng Krachan National Park, it is the only Iole bulbul in this area. Plumage-wise, it looks very similar to Grey-eyed Bulbul but usually appears slightly less yellowish overall, but there is no overlap between the distribution of Baker’s and Grey-eyed Bulbul to our current knowledge. It is more likely to be confused with Buff-vented Bulbul which it overlaps in the south. The call of Baker’s Bulbul is nearly identical to Olive Bulbul https://www.xeno-canto.org/336923.
The next species is the Grey-eyed Bulbul (I. propinqua). It is the default Iole bulbul when birding in the north, north-east and south-eastern Thailand. The race lekhakuni which is described to be found in the west is suggested to be a doubtfully recognisable subspecies of Olive Bulbul. Basically, the plumage of Grey-eyed Bulbul is probably unidentifiable from Baker’s Bulbul in the field (slightly more yellowish overall), but they have no overlap in the distribution. It can sometimes appear yellowish like Olive Bulbul but the iris is quite consistently pale brownish-grey. The best way to separate Grey-eyed from other Iole bulbuls is actually by the call. Grey-eyed Bulbul has a much flatter call compared to other species. The call of Grey-eyed Bulbul can be listened here https://www.xeno-canto.org/336887.
The last member of the genus is the Buff-vented Bulbul (I. crypta). It can be found only in the south where it overlaps with Baker’s Bulbul. It can be quite challenging to identify these two. Compared to Baker’s Bulbul, the Buff-vented Bulbul usually appears much less yellowish, especially on the underparts. The undertail coverts lack the rufous tinge as in Baker’s Bulbul, and often show a darker brown centre to each feather. The bill is also larger than in Baker’s, Olive and Grey-eyed Bulbul. The call is also similar to Baker’s Bulbul and can only be identified in the field with trained ears https://www.xeno-canto.org/336935.
For those who would like to read more about the Iole bulbuls, there’s an excellent study done by Sontaya Manawatthana et al. which can be accessed here “Phylogeography of bulbuls in the genus Iole (Aves: Pycnonotidae)“. It is also the study that suggested the split of Baker’s Bulbul (I. cinnamomeoventris), Charlotte’s Bulbul (I. perplexa), I. innectens, and Cachar Bulbul (I. cacharensis).