The genus Hypolais contains four Indian species so very like each other, and so difficult to separate one from the other, that it would, at first sight, seem preferable to unite them under one specific name. The slight differences that exist between them appear, however, to be correlated with geographical distribution, and so it is, on the whole, better to recognize these differences.
Hypolais moults twice in the year, but the moults are not accompanied by any visible change of colour in the plumage. The sexes are alike, and the young are less deeply coloured than is usual in birds of this family. They are all migratory, but their migrations are short and probably on the decline, as some few birds remain in their winter-quarters all the year through.
Hypolais resembles Sylvia very closely, and might perhaps be joined to it, but it has a sensibly larger bill, and on this account it is desirable to keep the two genera distinct.
In Hypolais the bill from the gape to the tip is longer than the middle toe and claw; there are three weak rictal bristles, and the supplementary hairs in front of them are obsolete, but still clearly visible with a lens. The first primary is small but not minute, and the third reaches to the tip of the wing. The tail is slightly rounded only. The colour of the plumage is very dull, and there is not a single bright spot or line to relieve it.
Key to the Species.
a. Primary-coverts reaching to about the middle of the first primary ; upper plumage olive-brown.
a1. Tail 2.2 or more ; bill from gape to tip .6 or more.
a2. Distance from tip of secondaries to tip of wing .4 ; from tip of first primary to tip of wing 1.1 to 1.25 ; wing 2.3 to 2.5…………….. H. rama, p. 391.
b2. Distance from tip of secondaries to tip of wing .55 to .65; from tip of first primary to tip of wing 1.3 to 1.5; wing 2.5 to 2.7…………….. H. pallida, p. 392.
b1. Tail under 2; bill from gape to tip .55, seldom more…………….. H. caligata, p. 393.
b. Primary-coverts extending over two thirds the Length of the first primary; upper plumage sandy brown…………….. H. obsoleta, p. 393.