Black-headed Penduline-tit (Remiz macronyx) - HBW 13, p. 69

Taxonomy: Aegithalus [sic] macronyx Severtsov, 1873, Shymkent (Chimkent), Syr Darya, south Kazakhstan.
Forms a species group with R. pendulinus, R. coronatus and R. consobrinus, and all four often combined as one species; sometimes present species considered conspecific with first of those, and the other two united as a separate species. Treatment as four distinct species based mainly on structural, as well as plumage and habitat, differences. Present species frequently hybridizes with R. pendulinus by N & SW shores of Caspian Sea; hybrids in N (mouth of R Ural) named as bostanjogli, and those in S as, respectively, loudoni and altaicus; last-mentioned sometimes treated as a valid race. Moreover, ssaposhnikowi believed probably to have originated as hybrid between nominate race of present species and race jaxarticus of R. pendulinus, although neither parent found within current range of ssaposhnikowi. Race neglectus sometimes synonymized with nominate. Birds from Aral Sea region described as race aralensis, but considered inseparable from nominate. Four subspecies currently recognized.