XI. Parus [ atricapillus]

The Willow Tits s.l. have turned out to be a touchstone for species systematics. The entire holarctic group associated with kleinschmidti and schiebeli, borealis and baicalensis, songarus, weigoldicus to restrictus, turneri, atricapillus, carolinensis and sclateri has been the object of intensive study for over a century. Hartert (1905: 376, Footnote 2) placed all of them except sclateri in a single species. But so far there is no agreement about their systematic status.