Key to Subspecies.
A. No black streak through eye.
a. Chin black.
a1. Sides of head all white.
a2. Darker above…………M. a. alba, p. 256.
b2. Paler above…………M. a. dukhunensis, p. 257.
b1 Sides of head mixed black and white…………M. a. persica, p. 258.
c1. Sides of head all black…………M, a. per sonata, p. 259.
b. Chin white…………M. a. baicalensis, p. 260.
B. A black streak running through the eye. …………M. a. ocularis, p. 261.
Hitherto the Pied Wagtail, M. lugubris^ has generally been accepted as a race of M. alba but Messrs. Lowe and Kinnear when working on the series of these two species in the British Museum came to the conclusion that there are two distinct groups of Wagtail, one the M. alba group, never acquiring black backs in the breeding-season, and the other, the M. lugubris group, which always acquires them. Each species contains geographical races and the division, as suggested by these two gentlemen, seems a natural one and, moreover, it accounts for seeming discrepancies in the overlapping of the breeding area of the two species and their subspecies.
Motacilla alba ocularis is also very doubtfully a subspecies of M. alba and its range in the breeding-season seems to overlap that of baicalensis. It is probable that when we learn more about it, it will have to be raised to the status of a full species. Again, the exact degree of relationship between M. l. aboides and M. l. leucopsis is very doubtful and some very careful collecting will have to be done during the breeding-season in Tibet before the question can be finally settled.