Systematics
Most authorities retain Poecile as a subgenus within the genus Parus, but the American Ornithologists' Union treats it as a distinct genus. 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).
The genus name has often been treated as feminine (giving the species name ending montana); however, this was not specified by the original genus author Johann Jakob Kaup, and under the International Code of Zoological Nomenclature the genus name must therefore be treated by default as masculine, giving the name ending montanus (del Hoyo et al. 2007).
At one time the Willow Tit was considered conspecific with the Black-capped Chickadee of North America due to their very similar appearance. In fact the Willow Tit, Black-Capped Chickadee, Marsh Tit and Carolina Chickadee are all very similar to one another in appearance.
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