Of the way in which Migration is performed there is still much to learn–but one thing is certain, all Birds do not migrate in the same manner. Some gather in flocks, great or small, others seem to accomplish their northward journey in pairs, or at any-rate arrive at their breeding-quarters already paired. Some undoubtedly voyage by night, others may be seen to travel by day. Of the Birds which in spring arrive unpaired, it is now incontestable that the males outstrip or precede the females. There is, moreover, equal diversity in the southward movements towards the close of summer and all through the autumn. Of some species the earlier broods disappear without attracting attention, and the later broods as well as the parents slip away almost as imperceptibly. In one remarkable case, that of the Cuckoo, the adults leave this country long before the young are fit to follow; but, in by far the greater number, the young start first, and are followed, often at an interval of some weeks, by their parents.[11] It is contended by many that of actual Migration we see very little, since it is constantly carried on at a height where the Birds are beyond our ordinary observation, and as regards some species this seems to be true. Moreover, it would seem that the longest flights are performed by night, and when the sky is clear, so that only in thick weather do the Birds come near enough to the earth to be heard–seeing them being of course impossible in the dark, though in a few cases they have been telescopically observed passing across the face of the moon. It is certain that many of the smaller land-birds gradually press {20}onwards prior to leaving our shores, but after that they may possibly betake themselves aloft to continue their journey. The speed at which Birds travel during Migration is a matter on which very diverse opinions have been and are held; but the highest estimates, such as those of the late Herr Gätke (who would allow even 150 miles an hour), can scarcely be otherwise than exaggerations; for there is no evidence of any but exceptional performances at such rates, and there is really no reason to suppose that Birds can fly faster at a higher elevation than at a lower. [Illustration: FIG. 6.–A Falcon. To shew the nomenclature of the external parts.] TERMINOLOGY.–The annexed figure explains the nomenclature of most of the outward parts of a Bird, but some further explanations may be given, as below:– {21}_Air-sacs._–Membranous receptacles, filled with air, in communication with the respiratory organs or passages. Pouches are often exaggerated air-sacs. _Alar bar._–A coloured bar across the wing (_ala_). _Allantois._–A vascular sack, growing from the hind-gut of the embryo and enclosed by the amnion; the two fuse together and form the Chorion, which lines the egg-shell, and takes upon itself respiratory functions.