39-2 



BIRD. 



The most singular part of the whole process of 

 flying- is, however, the ease with which the bird makes 

 the air a fulcrum, from which to leap, in that same 

 element, and that not only by repeated jerks or efforts, 

 which are characteristic of many of the smaller birds, 

 which thus leap from perch to perch, or from thicket 

 to thicket, but in the case of steady onward flight, in 

 which a few movements of the wings will sometimes 

 send the bird onward for many yards without any 

 other apparent movement ; and such appears to be 

 the impulse thus given, that the bird will sometimes 

 turn, and almost double back upon its former course, 

 by merely altering the inclination of the body and 

 wings, and without any new stroke or effort on the 

 part of these. The spring of the tiger, or the bound 

 of the antelope, though taken from the solid earth as 

 a fulcrum, and effected by what we consider as the 

 most powerful muscular action among the mammalia, 

 is a mere fraction in point of distance compared with 

 some of these gliding rushes of birds in the air ; and 

 it is worthy of remark, that birds cannot take these 

 motions when they rise directly from the ground, or 

 from their perch, but must have a certain quantity of 

 fluttering, or hurried wing motion, to bring them up to 

 their power. 



We know that when any thing is projected for- 

 wards the resistance from which it is projected must 

 be equul to the projectile force ; for no mechanical 

 force can act in one particular direction only, unless 

 by means of resistance in the opposite direction. 

 Consequently, when a bird gives itself an impulse in 

 the air, in which mere gravitation to the earth can 

 bear no part, and that cannot bear a part, unless the 

 direction is downwards, its body must strike the air 

 backwards with a force equal to that whicli impels 

 it forwards ; and, if the body of the bird had not a 

 means of resisting the reaction backwards, it might 

 move its wings ever so long or so vigorously without 

 advancing a single inch. That the bird derives the 

 greater part of that hold on the air, which enables it 

 to take as effective a leap from that element as it 

 could from a solid substance, and even more so, 

 directly by the action of the wings, is true ; but 

 there is no doubt that it receives considerable assist- 

 ance from the general muscular action. When 

 birds are on long and smooth flight, they also acquire 

 a momentum in proportion to their velocity, and the 

 difference between their specific gravity and that of 

 the air. In consequence of this momentum, they con- 

 tinue their progressive motion with much less effort ; 

 and the superiority of this momentum, in a rarer 

 atmosphere, may be an additional reason why they fly 

 high upon their long journeys. But there are many 

 birds which proceed by a succession of jerks or 

 leaps, in the pauses of which they are almost or 

 altogether at rest, and these birds can acquire but 

 little momentum, but must renew their whole impulse 

 at every jerk. These are, for the most part, birds 

 of low flight; and it is probable that the greater 

 resistance of the denser atmosphere is as advanta- 

 geous to them as the rarity of the upper strata is to 

 those species which fly with a momentum. Birds of 

 smooth flight also, however, often shoot onward with 

 great rapidity, after having hovered so long over the 

 same spot, as that all the momentum which they ac- 

 quired in arriving at that spot must be exhausted ; 

 and they must take the whole of their new velocities 

 from the resistance of the air. 



The flight of those birds which proceed by jerks 



affords a good illustration of the fact which has been 

 stated, of there being a buoyant or upward tendency 

 in the mere motion of the wings in flying. They do 

 not proceed upon a level, but by a series of flat ver- 

 tical parabolas, each as long as one of the jerks ; and 

 the motion is upward while the strength of the jerk 

 lasts, and downward as it weakens. The tail feathers 

 of such birds are also frequently flirted out horizon- 

 tally, so that the tail may either resist too great an 

 upward motion from the jerk, or act as a parachute 

 against the downward motion. 



The head and neck also come into action, both in 

 altering the lateral course of the bird, and in shifting 

 its centre of gravity, with reference to the central 

 line of action in the wings ; and the last alteration 

 has no inconsiderable influence in its ascents and its 

 descents. Indeed, a bird when flying is so mtich in 

 action in all its parts, that it is impossible to point 

 out the specific action of each. Thus we cannot ex- 

 plain the rationale of flight in any thing like a satis- 

 factory manner, and therefore though the operation 

 is not only possible, but performed habitually and 

 with ease, the precise mode in which it. is done still 

 remains one of the wonders of nature. 



We have made these general observations on the 

 operation of flying and the organs of flight, not only 

 because flying is peculiarly the motion of birds, and 

 the one from which they are named, but also to show 

 how much there is to be learned from operations of 

 nature, which we all have daily opportunities of ob- 

 serving, and yet upon which comparatively few of us 

 ever bestow a single thought. But it is not from 

 this species of motion alone that we can obtain any 

 thing like a knowledge of birds, neither can we 

 found wholly or chiefly upon it that classification 

 which is calculated to assist us in our inquiries. 



The general definition of a bird, at least as depend- 

 ent on its external appearance, is so simple and so 

 well understood, that the repetition of it would be 

 superfluous. It is a vertebrated animal ; and, be the 

 species what it might, no one would mistake it for 

 one of the mammalia, or a reptile, or a fish. There 

 have been some mistakes the other way, though they 

 have been but few : in the infancy of natural science 

 bats were considered as a sort of birds : and some of 

 the moderns, who have not quite arrived at the years 

 of scientific discretion in physiology, have gone 

 about to make a sort of bird of the ornithorhynchus. 



These mistakes show, what we find to be the fact 

 when we make the attempt, that the natural, or even 

 the satisfactory classification of birds according to an 

 artificial system, is no easy matter. The bats were 

 called birds on account of their flying membranes, 

 and the ornithorhynchus because of its mandibles, 

 which are something (but not very) like those of a 

 duck's bill, while both animals had all the essential 

 characters of true mammalia, though mammalia of 

 peculiar form arid habits. Thus it appears that 

 neither the bill nor the flight of birds can be taken 

 as the ground of a classification ; as little can the 

 feet ; and the digestive organs merely point out the 

 general kind of food, and not how or where it is 

 obtained. 



The feeding of birds cannot be made so good a 

 means of general distinction as that of the mammalia, 

 because many birds are so very miscellaneous in what 

 they eat that no one article can be considered as their 

 characteristic or leading food. And when we take 

 the three leading characters : the bill and digestive 



