Ixos

Taxonomy and systematics

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) will be split off into a distinct genus (for which no scientific name exists yet), while Hypsipetes is merged into the core group of Ixos.

Extant species

There are four extant species in the family Ixos:

Former species

Formerly, some authorities also considered the following species (or subspecies) as species within the genus Ixos: