As might be expected in a species which extends from Ceylon and the extreme North-West of India to China and the Malay Islands, there are several geographical races all recognized as different by the older naturalists and named and later on suppressed when they were found not to constitute good species. Now, however, we realize what subspecies are, once they are more recognized.
It is curious that the females vary far more than do the males and it is principally through them that the forms are most easily recognizable.
Key to Subspecies.
A. Plumage all black and white.
a. Three outer pairs of tail-feathers white, fourth pair almost or
entirely so; culmen about 18 mm. C. s. saularis, p. 113.
b. Third pair of tail-feathers much marked with black, fourth
pair almost or wholly black ; culmen 19 to 20 mm C. s. musicus, p 114.
c. Third pair of tail-feathers white, fourth pair heavily marked with
black; culmen 16 to 17 mm. .. C. s. ceylonensis, p. 115.
d. Third pair of tail-feathers marked with black and fourth pair heavily
marked; culmen 18 mm C. s. andamanensis, p. 116.
B. Plumage grey, or brown, and white.
e. Abdomen almost pure white.
a1. Four outer pairs of tail-feathers white or nearly so; culmen
about 18 mm C.s. saularis, p. 113.
b1. Third pair of tail-feathers marked with black, fourth pair almost or wholly black; culmen 19 to 20 mm…………C.s. mtisicus, p. 114.
c1 Third pair lightly, fourth pair of tail-feathers heavily marked with black; culmen 16 to 17 mm……………..C. 8. ceylonensis, p. 115.
f. Abdomen heavily marked with rusty; culmen 18 mmC.s. andamanensis, p. 116.
Size, except as regards the bill, does not help in any way to separate the species, as when large enough series are obtained the averages are much the same. Chinese and Ceylon birds average rather larger; Assam, Burmese and Hainan birds a little smaller.