Motacilla Linn., Syst. Nat., 10th ed. i, p. 184 (1758). Type, Motacilla alba Linn
The genus Motacilla contains the true Wagtails, found over the whole of the Old World.
In Motacilla the plumage is plain, being devoid of all streaks, mottlings or spots on the upper surface; the sex:es are practically alike but the difference between the Summer and Winter plumage in most of the species is very great. The Pied Wagtails are constantly undergoing a change of colour and it is hardly possible to find two birds at the same date in the same state of plumage. This makes it very difficult to describe them for identification. The Yellow Wagtails do not undergo such complete changes as the Pied but their similarity to one another is so great in Winter and in immature plumage that their recognition is even more difficult.
A very excellent little paper by Ticehurst, which appeared in the Bombay Natural History Journal (vol. xxx, pp. 1082-1090, 1922), will be found of great assistance in discriminating between the various races, though space forbids their being dealt with in a similar manner in the present volumes.
The Wagtails have the wing and tail about equal in length or the tail decidedly shorter and have been often divided into two genera, Motacilla and Budytes, the former including the long-tailed Pied group and the latter the shorter-tailed Yellow group. To me there does not seem any necessity to make this division and I, therefore, retain all the Wagtails in Motacilla.
Key to Species.
A. Hind claw much curved and shorter than hind toe.
a.Plumage black, white and grey.
a1. Back never black in Summer…………M. alba, p. 255.
b1. Back black in Summer…………M. lugubris, p. 261.
b.Plumage largely green and yellow…………M. cinerea, p. 265.
B. Hind claw little curved and much longer than hind toe.
c. Supercilium and forehead never both yellow.
c1. Grown never black, a supercilium always present…………M. flava, p. 267.
d1. Crown always black, or black with green¬ish fringes," supercilium not present…………M. feldegg, p. 271.
d. Both forehead and supercilium bright yellow…………M. citreola, p. 273.