There are four species in the genus Remiz: Eurasian Penduline Tit (R. pendulinus), Black-headed Penduline Tit (R. macronyx), White-crowned Penduline Tit (R. coronatus) and the Chinese Penduline Tit (R. consobrinus). This latter two are clearly separate species, but there is little genetic differentiation between Eurasian and Black-headed Penduline Tit. Observations of birds with intermediate morphology near Topar (Kazakhstan) suggests that these species are interbreeding (Barani‐Beiranvand et al. 2017).
Barani‐Beiranvand, H, M Aliabadian, M Irestedt, Y Qu, J Darvish, T Székely, RE Van Dijk, PG Ericson. (2017). Phylogeny of penduline tits inferred from mitochondrial and microsatellite genotyping. Journal of Avian Biology, 48:932-940.