The genus name has traditionally been treated as feminine (giving name endings such as cincta); however, this was not specified by the original genus author Johann Jakob Kaup, and under the ICZN the genus name must therefore be treated by default as masculine, giving name endings such as cinctus.
See Varied Tit as for why present data suggests placement in Poecile instead of Sittiparus.
This is supported by mtDNA cytochrome b sequence analysis; within Poecile, the Willow Tit is part of a group of Eurasian "typical chickadees" that includes also the Marsh Tit, Père David's Tit, and the Songar Tit (Gill et al. 2005, del Hoyo et al. 2007).