« ПредишнаНапред »
Why should it fly at the highest pressure when there is no need for such a proceeding ? What wild animal known to us in Britain exercises its fullest powers for more than the briefest time, save under the stimulus of fear? I think it quite likely that the mallard we come upon suddenly calls into play greater exertion during the first hundred yards of its flight than it does during a thousand yards when no longer under the influence of fear. The difference between the energy exerted by those golden plover which dash by us at lightning speed, compared with the energy exerted by them when covering the same distance during their migration flight, is very likely about the same as the difference between the energy we exert when walking and when running. Who ever saw a greenfinch travel at any other time as it travels when chased by a hawk ? Assuredly no one. The bird, knowing it to be a question of life or death, flies, perhaps, three times as swiftly as it ever flies in the ordinary course of things, or as any human being can make it fly, even when he fires off a gun only a dozen yards away. Under special circumstances—the influence of fear—the greenfinch exerts energy much greater than it exerts at any other time, and flies with much greater speed ; just as under special circumstances, the force of the instinct to cover the journey
in the briefest time possible, before the wind or the weather changes, the hooded crow exerts energy at the times of migration which it never thinks of exerting, and probably could not exert, at any other time.
There can be little doubt that the instinct of exerting great and prolonged energy when making the migration journey is built upon the fear of weather changes. The shorter time the journey takes, the less is the risk of a sudden adverse gale. Birds must always face this risk. Every year large numbers perish through the springing up of gales, which drive them from their right course or keep them battling against the wind till they succumb to exhaustion. Considering the changeability of the weather, the disastrous effects which an adverse wind has upon the flight of migrating birds, and the way in which necessity and experience have produced bird instincts, it would be a matter of considerable wonderment if migratory birds had not evolved an instinct to make a huge effort, to exert themselves to their very highest powers to effect their uncertain passage from shore to shore in the shortest possible time. Nature has lent physical assistance to this instinct, and thus the migrating bird has not only the potent instinct to push on with might and main, but also the special
physical power, special to those two periods of the year, to put the instinct into effect.
The carrier pigeon has often been put into the box as a witness against the high speed of migrating birds; but his evidence is not worth much. In the first place, if he ever possessed the migration instinct, civilisation has killed it, and it is only under the influence of this instinct that the wild bird makes a lengthy journey at abnormal speed. In the second place, even if he have in a subdued form the instinct of migration, his civilised life places him outside the conditions which brace up the system of the wild migratory bird twice in the year, and bring into active play the sub-instinct to the migration instinct proper —that of exerting his braced-up system to its highest powers in order to complete a risky journey in the minimum of time: he has not the fear of the migrating bird, for, whereas he can rest during a protracted gale, the migrating bird stands a very good chance of being drowned. In the third place, the carrier-pigeon never attains an altitude approaching to that assumed by migrating birds. Secondly—it may
be for this reason alone; only following the possibilities of a matter about . which we know very little indeed—the migrating bird by flying at a great height gains a considerable
advantage in wing power over the same bird flying near the earth. This again critics contest, and they argue on strong grounds. To take an example: 'It has been repeatedly asserted that birds attain a rate of over two hundred miles an hour by flying at such a height that the air, being rarefied, offers only a slight resistance to the forward motion of their bodies. A very slight knowledge of mechanical principles would show that the resistance of the air to their wings, required for propelling them forward, must diminish in exactly the same degree' (I might remark that the writer of these words apparently loses sight of, or is ignorant of, the fact that resistance is not merely afforded by the air during a bird's flight, but by the air, the set of the wind, and the force of gravitation combined). But the scientific world said on the ground of mechanical principles that a railway train could never keep on the metals; and we are still a long way from knowing what mechanics will and will not allow.
That the migrating bird does actually derive advantage from flying at a great height seems indubitable on the evidence of the bird itself. If it did not thus derive advantage, why should it uselessly expend the energy necessary to climb to such a height? It has been argued that the reason why such a height is
attained by migrating birds is that by placing themselves at a great elevation they are enabled to see the coast for which they are making sooner than would be the case if they flew at a lower level, and are thus able to minimise the risk of losing their way, as well as having the advantage of longer daylight should their journey happen to be delayed. But woodcock start their journey at dusk and finish it long before dawn, and not only this, but they reach us more numerously on a misty night than on a clear one. I believe, rightly or wrongly, that the sense of orientation is developed to such a degree in birds possessing the instinct of migration for long distances over sea in the lines taken by their ancestors for countless generations, that they need no further assistance whatsoever in determining the trend of their passage ; that, gales apart, it guides them unerringly to their destination ; and that any guidance by vision is quite unnecessary to them. How else do woodcock, flying by night, find their way year after year to the same points on our coast ? We could scarcely credit them with a knowledge of the position of the constellations even if they made a practice of never crossing the sea save on clear nights.
Fully to discuss what the possible advantage, maintaining on the evidence of the birds themselves