|The Mountain Bulbul (I. mcclellandii) is provisionally placed in Ixos, until its relationships are resolved.|
|[[#Species, systematics and taxonomy|7 species, but see text]]|
- Nicobar Bulbul, Ixos nicobariensis
- Mountain Bulbul, Ixos mcclellandii
- Streaked Bulbul, Ixos malaccensis
- Sunda Bulbul, Ixos virescens (type species)
Systematics and taxonomy
The genus as traditionally delimited is not monophyletic; the Brown-eared Bulbul is placed in Ixos, but in fact it is better separated in the monotypic genus Microscelis. The Philippine Bulbul is sometimes placed in Ixos but otherwise (and probably more appropriately) in Hypsipetes, where the Sulphur-bellied, Zamboanga and Streak-breasted Bulbuls might also belong.
The Mountain Bulbul and Sunda Bulbul – formerly H. mcclellandii and H. virescens – are provisionally placed in Ixos until the genus limits in bulbuls are firmly established. They are often included in Hypsipetes but this is an old error that was promoted in modern times by the Sibley taxonomy. In fact, I. virescens is the type species of Ixos, and though some sources advocate a complete merger of the two genera – and even the entire "Hypsipetes group" of bulbuls, which also includes Hemixos, Iole, Microscelis and Tricholestes –, being the oldest genus name Ixos would apply to all of them, rather than Hypsipetes as is often believed.
The most drastic course hardly appropriate however, since Alophoixus and Setornis cannot be excluded from the "Hypsipetes group", and an all-out merge would turn the resultant "genus" Ixos into an ill-defined "wastebin taxon". The erroneous inclusion of I. virescens in Hypsipetes has caused the Nicobar Bulbul to be listed under its invalid junior synonym H. nicobariensis rather than the valid names H. virescens or I. nicobariensis.
The Streaked Bulbul, meanwhile, seems to be closer to Hemixos and perhaps Iole and maybe the Hook-billed Bulbul (Setornis criniger) than to Hypsipetes. Thus, given the lack of study of the Sunda Bulbul's affiliations, Ixos might eventually be reduced to the core group around I. virescens and kept well distinct from Hypsipetes. Or it may be that some species (such as I. malaccensis) are be split off into a distinct genus (for which no scientific name exists yet), while Hypsipetes is merged into the core group of Ixos.
- Gregory, Steven M. (2000): Nomenclature of the Hypsipetes Bulbuls (Pycnonotidae). Forktail 16: 164-166. PDF fulltext
- Moyle, Robert G. & Marks, Ben D. (2006): Phylogenetic relationships of the bulbuls (Aves: Pycnonotidae) based on mitochondrial and nuclear DNA sequence data. Mol. Phylogenet. Evol. 40(3): 687-695. doi:10.1016/j.ympev.2006.04.015 (HTML abstract)
- Pasquet, Éric; Han, Lian-Xian; Khobkhet, Obhas & Cibois, Alice (2001): Towards a molecular systematics of the genus Criniger, and a preliminary phylogeny of the bulbuls (Aves, Passeriformes, Pycnonotidae). Zoosystema 23(4): 857-863. PDF fulltext
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