But nevertheless, a very considerable number of our British birds can be more or less easily distinguished during flight--sometimes by the manner of that flight, sometimes by characteristic markings, sometimes by the notes they utter; and these are briefly summarised in this Chapter. [Illustration: 1. Swallow. 2. House Martin. 3. Swift. 4. Sand Martin. 5. Pied Wagtail. 6. Grey Wagtail. 7. Yellow Wagtail. 8. Chaffinch. 9. Goldfinch. 10. Linnet. 11. Greenfinch. 12. Bullfinch. ] When it is realized that no less than 475 species, and sub-species, of British birds are now recognized, it will be apparent that it would be impossible to do more than briefly epitomise the commoner species, and some of these, like the robin, and the wren, need no interpreter. The aim of this Chapter is primarily to give, as far as possible, the salient features of our commoner native birds, as seen during flight. But some species merely "flit," from one place to another, and that so rapidly that no details of coloration can be distinguished. They can only be examined at favourable, and often fleeting moments, when at rest, and clear of foliage. Only such as are frequently encountered are included here. To attempt more would be to lead to confusion. Enough, it is hoped, will be said to help the beginner. Experience will soon lead to an ever increasing proficiency--and with this will come an ever increasing conviction that the identification of birds, during flight, is an extremely difficult task. Whoever essays it should, whenever possible, supplement his efforts by the aid of a pair of good field-glasses. These, indeed, are indispensable. The small perching birds are, perhaps, the most difficult to name at sight, and this because their flight presents so little to distinguish one species from another. All fly with rapid wing-beats, alternating with a period during which the wings are practically closed, causing the body to travel forward on a rapidly descending curve in the interval between the wing-beats. This gives rise to what is known as an "undulating" flight. But the large passerines, like the crows, differ conspicuously in their method of progress. With them the wing beats relatively slowly, so that its shape can be readily seen; and their course is direct--hence the familiar saying "straight as the crow flies." Further, the inner webs of the outer primary quills are, what is called "emarginate," that is to say, the width of the web is suddenly reduced towards the tip of the feather, so that the outstretched wing has a conspicuously fringed appearance, as may be seen at a glance at the beautiful pen-and-ink sketches on another page. The eagles and falcons have similar emarginations.