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Show 222 DARWINISM CHAP. Amona birds these recoanition marks arc especially numerout and ~uggcstive. bSpecics which inhabit open districts are usually protectively coloured; but they generally possess some distinctive markings for the purpose of bc~ng easily recognised by their kind, both when at rest and durmg flight. Such are, the white bands or patches on the brea t or belly of many birds, but more especially the head and neck markings in the form of white or black caps, collar , eye-marks or frontal patches, examples of which arc seen in the three species of African plovers figured on page 2 21. Recoo·nition marks during flight are very important for all birds which congrcga,te in flocks or which migrate together ; and it is essential that, while being as conspicuous as possibl e, the marks shall not interfere with the general protective tiHts of the species when at rest. Hence they usually consist of well-contrasted markings on the wings and tail, which arc concealed during repose but become fully visible when the bird takes flight. Such markings arc well seen in our four British species of shrikes, each having quite different white marks on the expanded wings and on the tail fc:tthcrs; ancl the same is the case with our three species of Saxicola-the stone-chat, whin-chat, and wheat--ear-which arc 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 tho . arne districts, well illustrates these specific recognition marks. Though not very greatly different to our eyes, they arc no doubt amply so to the sharp vision of tho birds themselves. Besides the white patches on the primaries here shown, the secondary feathers arc, in some cases, so coloured as to afford very distinctive markings during flight, as seen in the central sccoru::lary quills of two African coursers (Fig. 21 ). Most characteristic of all, however, arc the varied markings of the outer tail-feathers, whose purpose is so well shown hy 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, VIII ORIGIN AND USES OF COLOUR IN ANIMALS 223 Fm. 20.-<Edicnemus vermiculatus (above). ill. senegalensis (below). |