BIRDS IN GENERAL. 



3 



The next object that comes under consider- 

 ation, in contemplating an animal that flies, 

 is the wing, the instrument by which this 

 wonderful progression is performed. In such 



wings also move in silence, or when they are brought 

 into such rapid action as to produce a sort of noise, it is 

 a low and muffled rustle, and does not ring out, so that 

 the largest butterfly or moth gets along much more si- 

 lently than the gnat. We may add, as a further in- 

 stance of the same kind, that the bats when they fly are 

 always obliged to winnow the air with their flying mem- 

 branes, something in the same way as naked winged in- 

 sects do, though the flight of bats, unless when they are 

 agitated, is comparatively noiseless. So also those rep- 

 tiles which fly by means of membranous appendages are 

 obliged to flutter these very much in proportion to the 

 rate of their progressive motion. 



Now, the difference of action in these two textures of 

 wings in the other classes of animals, shows us the ad- 

 vantages which birds derive from their feathery cover- 

 ing and feathery organs of flight. These feathers, even 

 to the minutest fibre on the plumes or webs, are tubular, 

 consisting of only a thin film of solid matter, filled with 

 air within, though strengthened by partitions of cellular 

 substance, more or less close together, recording to the 

 strain which the feathers have to bear. From the mode 

 in which the feathers and all their parts are laid upon 

 the bird, it presents a smooth surface upwards and for- 

 wards, so that the animal can move in either of these di- 

 rections with very little resistance from the friction of 

 the air. When it moves in either of them, the resis- 

 tance of friction does not increase so rapidly as the rate 

 of motion ; because the pressure smooths the feathers, 

 and causes the air to take less hold of them. This pro- 

 perty, which arises in part from the texture of the up- 

 per surface of the feathers, but chiefly from the way in 

 which they are formed and placed, is of equal service to 

 birds when they must perch or otherwise remain at rest 

 so as to abide the blast, as when they fly exposed to it. 

 Perching or flying, when a bird is in the wind it always 

 faces the current, and thus offers the least resistance 

 both by its form and its feathers. 



When, however, the feathers are taken in the oppo- 

 site directions, they offer as much increase of resistance 

 as they offer diminution when they are taken above or 

 in front. The wings are always more or less hollow on 

 the under sides, and they take hold of the air by millions 

 of fibres, so that a bird with its flying feathers on the 

 stretch, would fall much more slowly than one would 

 suppose from the difference between its specific gravity 

 and that of the air. 



The resistance which all the feathers on the body of 

 the bird offer to motion backwards is still greater ; and it 

 increases with the force which tends to move the ani- 

 mal in that direction. The instant that it begins to be 

 driven backwards, so that a current against its body is 

 produced, the points of the feathers rise and take the 

 wind with so many fibres, that the resistance is very si- 

 milar to that made by a scaly fish, when one attempts to 

 draw one of these by the tail ; and every one who has 

 angled, and accidentally caught even a common trout in 

 that way, knows that an ounce weight is as difficult to 

 land when so hooked as a pound weight is when hooked 

 by the head. But the feathers of birds rise much more 

 in proportion than the free edges of the scales upon any 

 fish, and they are every way as well formed for " hold- 

 ing on" in the air, as those are for holding on in the 

 water. Thus the bird may be said to resist motion 

 backwards in the air, by throwing out the point of each 

 feather like the " fluke" of an anchor. 



The bird, when its habit is to be much on the wing, 

 is all over adapted for flight; and the system of its me- 

 chanics, if we could fully comprehend it, would certain- 



birds that fly, they are usually placed at that 

 part of the body which serves to poise the 

 whole, and support it in a fluid that at first 

 seems so much lighter than itself. They an- 



ly be the most curious, and far from the least instruc- 

 tive, in the whole of the animal kingdom. 



The buoyancy, as well as the upward motion, is not 

 very difficult to understand, because the wing, from its 

 general form, and the structure of the feathers, rises* 

 with much less effort than it descends. Thus the con- 

 stant tendency of the powerfully winged bird is to mount 

 upwards; and on this account the firmest bird, that 

 which with the same volume of body and extent of 

 wings has the greatest specific gravity, is the best flyer, 

 flies more steadily, and apparently with less effort. 

 This must of course have a limit ; because, leaving the 

 incapacity of breathing out of the question, no bird could 

 fly in a vacuum, and thus there must be a certain den- 

 sity of air which is the best adapted for the flight of any 

 given species of bird. This appears, even in the case of 

 heavy birds, to be considerably less than the density of 

 the mean level of the earth's surface. Eagles are heavy 

 birds, even for their powerful wings, and yet they are 

 high fliers, even when their abodes are at great eleva- 

 tions in the mountains. All birds which take long 

 flights fly high, whatever may be their other habits. 

 Wild geese, herons, all birds indiscriminately " take the 

 sky" when they set out upon long journeys. In some, 

 this may be in part done to avoid enemies or obstacles, 

 but the habit is too general for being accounted for upon 

 any principle, save that the high flight is the less fatigu- 

 ing. Even rooks may be observed to adjust the height 

 of their daily excursions from the rookeries to the dis- 

 tance at which the pasture upon which they are to feed 

 lies ; and the swallow tribe wheel about far more rapidly 

 and gracefully when they hawk high before rain, than 

 when they skim the surfaces of the pools in fine weather. 

 If we may judge from their appearance when we see 

 them on the wing (the only means we have of judging), 

 it appears that birds, when they are not in search of any 

 thing upon the ground near them, mount up till they 

 come to that density of atmosphere which is best suited 

 to their weight and wings, and then continue onwards. 

 There may be another reason : those upper regions to 

 which the birds ascend on their long nights are in a 

 great measure exempted from the momentary gusts and 

 squalls which war upon the surface under them. 



The circulation of blood is, as has been hinted already, 

 more rapid in birds than in the mammalia, which agrees 

 with the greater violence and longer continuance of some 

 of their actions. But though these more violent actions 

 such as coursing on two feet, as fleetly as antelopes do on 

 four, and with the aid of the flexible spine and its mus- 

 cles, as in the ostrich plunging into the water like the 

 gannet or the cormorant dashing through that element 

 like the divers cleaving the air beyond comparison 

 with all terrestrial speed, as in the falcon, the swift, or 

 the pratincole, or breasting the tempest with the majesty 

 of the eagle require, and are furnished with, a supply 

 of blood proportional to the waste which their great 

 energy must occasion; yet they are by no means so well 

 suited to an equally rapid breathing by means of lungs. 

 But the application of renovating air to the blood must, 

 in all animals, be proportional to the circulation, and, 

 among vertebrated animals, it is only the reptiles and 

 fishes which have the temperature low and the circula- 

 tion lagging, and which spend much of their time in a 

 state of comparative inaction, that can carry on their 

 systems in a healthy state with only a partial aeration of 

 the blood. 



If the subject is considered according to our plans in 

 contriving and executing, there is thus a difficulty to be 

 overcome in the case of the birds, similar to which no- 



