14 FUNCTIONS OF BIRDS. 



As birds are continually passing among the hedges and thickets, their eyes are de- 

 fended from injury by a membrane, which can at pleasure be drawn over the whole 

 eye like a curtain. 'This is neither opaque nor wholly pellucid, but is somewhat 

 transparent. In birds we find that the sight is much more piercing, extensive, and 

 exact, than in the other orders of animals. The eye is large in proportion to the 

 bulk of the head. This is a superiority conferred upon them not without a corres- 

 ponding utility; it seems even indispensable to their safety and subsistence. "Were 

 this organ dull, or were it, in the least degree, opaque, the rapidity of their motion 

 would expose them to the danger of striking against various objects in their flight. 

 Jn this case their celerity, instead of being an advantage, would become an evil, and 

 their flight would be restrained by the danger resulting from it. Indeed, we maj 

 consider the velocity with which an animal moves, as a sure indication of the perfec- 

 tion of its vision. 



Birds respire by means of air-vessels, that are extended through their whole body, 

 and adhere to the under surface of the bones. These, by their motion, force the ait 

 through the true lungs, which are very small, seated in the uppermost part of the 

 chest, and closely braced down to the back and ribs. The use of this general diffu- 

 sion of air through the bodies of birds, is to prevent their respiration from being 

 stopped or interrupted by the rapidity of their motion through a resisting medium. 

 The resistance of the air increases in proportion to the celerity of the motion ; and 

 were it possible for a man to move with swiftness equal to that of a Swallow, the 

 resistance of the air, as he is not furnished with reservoirs similar to those of birds, 

 would soon suffocate him. 



Some species of birds are confined to particular countries ; others are widely dis- 

 persed ; and several change their abode at certain seasons of the year, and migrate 

 to climates better suited to their temperament or mode of life than those which they 

 leave. Many of our own birds, directed by a peculiar and unerring instinct, retire, 

 before the commencement of the cold season, to the southern districts, and again 

 return in the spring. The causes usually assigned for migration are, either a defect 

 of food, or the want of a secure and proper asylum for incubation, and the nutrition 

 of their offspring. 



It appears from very accurate observations, founded on numerous experiments, 

 that the peculiar notes, or sony, of the different species of Birds, are acquired, and 

 are no more innate than language is in man. The attempt of a nestling bird to sing, 

 may be compared with the imperfect endeavor of a child to talk. The first essay 

 seems not to possess the slightest rudiments of the future song ; but, as the bird 

 grows older, and stronger, it is not difficult to perceive what he is attempting. 

 Whilst the scholar is thus endeavoring to form his song, when, he is once sure of a 

 passage, he commonly raises his tone ; but when unable to execute the passage, he 

 drops it. What the nestling is thus not thoroughly master of, he hurries over; 

 lowering his tone, as if he did not wish to be heard, and as if he could not yet satisfy 

 himself. A common Sparrow, taken from the nest when very young, and placed 

 near a Linnet and Goldfinch, adopted a song that was a mixture of the notes of these- 

 two. Three nestling Linnets were educated, one under a Sky-lark, another under a 

 Wood-lark, and the third under a Tit-lark ; and, instead of the song peculiar to their 

 own species, they adhered entirely to that of their respective instructors. A Linnet 

 taken from the nest when about three days old, and brought up in the house of Mr 

 Matthews, an apothecary, at Kensington, having no other sounds to imitate, almost 

 articulated the words " pretty boy ;" and a few other short sentences. The owner ol 

 ibis bird said, that it had neither the note nor the call of any bird whatever. It 

 died in the year 1772. 



These, and other well-authenticated facts, tend to prove that Birds have no innate 

 notes, but that, like mankind, the language they first learn after they come into the 

 world, is generally that which they adopt in after life. It may, however, seem unac- 

 countable, why, in a wild state, they adhere so steadily as they do to the song of 

 their own species only, when the notes of so many others are to be heard around 

 them. This evidently arises from the attention that is paid by the nestling bird to 

 the instructions of its own parent only, and it is generally disregarding the notes of 

 all the rest. Persons, however, who have an accurate ear, and have studied the 

 notes of birds, can very often distinguish some that have a song mixed with the 

 notes of other species. 



ITie food of birds is of course very different in the different kinds. Some are 



