of the zebra and the quagga
are probably due to the same cause, as may be the singular crests and
face-marks of several of the monkeys and lemurs.[85]

[Illustration: FIG. 19--Recognition marks of three African plovers.]

Among birds, these recognition marks are especially numerous and
suggestive. Species which inhabit open districts are usually
protectively coloured; but they generally possess some distinctive
markings for the purpose of being easily recognised by their kind, both
when at rest and during flight. Such are, the white bands or patches on
the breast or belly of many birds, but more especially the head and neck
markings in the form of white or black caps, collars, eye-marks or
frontal patches, examples of which are seen in the three species of
African plovers figured on page 221.

Recognition marks during flight are very important for all birds which
congregate in flocks or which migrate together; and it is essential
that, while being as conspicuous as possible, the marks shall not
interfere with the general protective tints of the species when at rest.
Hence they usually consist of well-contrasted markings on the wings and
tail, which are concealed during repose but become fully visible when
the bird takes flight. Such markings are well seen in our four British
species of shrikes, each having quite different white marks on the
expanded wings and on the tail feathers; and the same is the case with
our three species of Saxicola--the stone-chat, whin-chat, and
wheat-ear--which are thus easily recognisable on the wing, especially
when seen from above, as they would be by stragglers looking out for
their companions. The figures opposite, of the wings of two African
species of stone-curlew which are sometimes found in the same districts,
well illustrates these specific recognition marks. 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