Though not very greatly different to our eyes, they are no doubt amply so to the sharp vision of the birds themselves.
Besides the white patches on the primaries here shown, the secondary feathers are, in some cases, so coloured as to afford very distinctive markings during flight, as seen in the central secondary quills of two African coursers (Fig.

21).
[Illustration: FIG.

20 .-- Oedicnemus vermiculatus (above).

Oe.
senegalensis (below).] Most characteristic of all, however, are the varied markings of the outer tail-feathers, whose purpose is so well shown by their being almost always covered during repose by the two middle feathers, which are themselves quite unmarked and protectively tinted like the rest of the upper surface of the body.

The figures of the expanded tails of two species of East Asiatic snipe, whose geographical ranges overlap each other, will serve to illustrate this difference; which is frequently much greater and modified in an endless variety of ways (Fig.