B O T T L E - T I T. 



f.13 



one in confinement a large spider's nest, it was 

 evidently at a loss to know how to pull out the 

 eggs, tried to nibble out a small portion, and lay the 

 remainder beside it on a perch, but was unsuccessful, 

 and on returning to look at it after a few minutes' 

 absence, it was found still carrying it about in its bill ; 

 but, on fastening it to a perch, it was demolished in 

 an instant. Its strong muscular neck seems peculiarly 

 adapted for pulling any thing upward by force, and it 

 is thus that, in a state of nature, its food is mostly 

 obtained. 



In a recent publication*, after describing the minute 

 insects on which the bottle-tit appears chiefly to sub- 

 sist, some highly interesting remarks are introduced, 

 on the relative degree of acuteness of vision in this 

 and other species ; from which it would appear that, 

 in all probability, the sight of the bottle-tit is more 

 peculiarly adapted for distinguishing very minute ob- 

 jects, than that of any other bird whatever. " It is 

 probable," observes the writer, " that though this bird 

 is a little larger than the gold-crested wren," the 

 smallest of British birds, " it feeds on smaller prey. 

 From the centre of the eyes to the tip of the bill 'is 

 only four-tenths of an inch, while that in the golden- 

 crested wren is one half more, or six-tenths. Hence 

 it is probable that, in its vision, the long-tailed tit is 

 the most microscopic of birds. Now it accords with 

 analogy and with observation, as far as that can be 

 carried, that the shortest focal length of the eye, that 

 at which minute objects can be distinctly seen, is 

 never much longer (if longer at all) than the distance 

 from the eye to the tip of the bill ; and nearer than 

 that would be useless. 



" Hence we have a very easy means of judging of 

 the relative power of vision in those birds which feed 

 upon very small substances, and which, being," at 

 least comparatively, " hard billed, as the tits are, or 

 not having the bill covered with a sentient membrane, 

 or the nasal groove continued to the point of it, as is 

 the case with, that most singular of all birds the 

 apteryx, or wingless bird of New Zealand, must feed 

 wholly or chiefly by sight. 



" In the average of human eyes, of good formation, 

 and in a healthy state, the common focal distance for 

 small objects, such as for reading very small print, is 

 about seven inches and a half; and the microscopic 

 distance, such as that at which a very delicate engrav- 

 ing is examined (and that requires' a little time for 

 the adjustment of the eye), is about one-third less, or 

 five inches. The bird must often strike its prey 

 without any time for adjustment, and therefore the 

 distance from the eye to the bill may be taken as the 

 ordinary short focal length ; but call it microscopic, 

 and the magnifying power in line being inversely as 

 the focal length, the lineal dimensions of an object 

 seen by the long-tailed tit will be to that of the same 

 seen by the human eye, as five inches to four-tenths 

 of an inch, or as twelve and a half to one. The sur- 

 face will of course be as the square, or as 156 to 1, 

 and the body or solid of an object, as the cube, or 

 I953 to 1. Thus an aphis or any other small insect 

 that can come wholly within the field of vision, will 

 appear very nearly as large to the long-tailed tit as 

 two thousand of the same would to the human eye. 



" The microscopic power of the eyes of 

 those little birds which seek for minute prey on the 

 bark of trees is, therefore," (the same author afterward 



" Feathered Tribes of the British Islands," by Robert Mudie. 

 Article LO.VO-TAILED TIT. 



continues, and we cannot do better than quote his 

 observations,) " as wonderful as the telescopic range 

 of the eyes of eagles and other birds which soar aloft, 

 and scan a horizon of miles ; and not the least won- 

 derful part of the whole matter is the ease with which 

 the eye changes from" telescope to microscope. The 

 eye of the eagle which can discern the motion of a 

 small quadruped at the distance of more than a mile, 

 can shorten its focus, so as to be keen and perfect at 

 the distance of a few inches ; and the tit, to whose 

 near vision the eggs of flies must appear as large as 

 musket bullets do to us, feels no difficulty at seeing a 

 bush at the distance of more than a hundred yards. 

 We may admire, but we cannot imitate that wonder- 

 ful mechanical skill which can, in an organ not alto- 

 gether larger than a pepper-corn, produce so many 

 focal lengths in the same crystalline lens, instantly 

 adapt the retina, or sentient tissue, to all of them, 

 manage the admission of the proper light by the iris, 

 and remove the colours and correct the aberrations 

 by the other humours ; all, as it were, in an instant, 

 without effort, and (to use the common though unphi- 

 losophical terms) without will or consciousness on the 



part of the owner 



" Eyes have no doubt a wonderful flexibility 



in their focal length, so that the same eye is to a con- 

 siderable extent both a telescope and a microscope. 

 But still their flexibility or power of adaptation in 

 that way has a limit; and as the focal length of the 

 long-tailed tit's eye is not more than a twelfth that of 

 the human eye, we may suppose that it cannot see so 

 well at the distance of half a mile as man does at the 

 distance of twelve miles." 



Still, probable however as this last supposition 

 may be in the case of the bottle-tit, the gold-crests, 

 and a few other species, it certainly will not apply 

 alike to the generality of small birds. The power of 

 telescopic vision in most birds, even the smaller kinds, 

 is very much greater than most naturalists seem to 

 be aware of; and in this class of animals generally, 

 as is indicated by the very flattened form of the crys- 

 talline lens, and as might perhaps be anticipated from 

 the comparatively very large size of the eye, in pro- 

 portion to that of the head, appears very greatly to 

 exceed what it is in any other class. The great ex- 

 tent also to which diurnal birds can alter the apparent 

 size of the retina, and consequently regulate the 

 degree of light admitted into the eye, is very worthy 

 of attention ; and this may be easily observed by 

 holding in the hand a whitethroat, or some other 

 small species with pale irides, for it is curious that, 

 when struggling to make their escape, they keep 

 continually dilating and contracting the iris, often to 

 its utmost ; and after witnessing the extent to which 

 they do this, we can no longer feel surprise that the 

 eyes of diurnal birds are never dazzled by intense 

 light. Thi? eagle has been always famed for gazing 

 with undazzled vision full at the sun, but she does this 

 in common with every little bird ; the sparrow, or 

 the robin, or the common fowl, can face the sun as 

 well as he ; and this not, as is the commonly received 

 opinion, by shielding the eye with the nictitating 

 membrane, but simply by contracting the pupil, as 

 any one, who will take the trouble to observe a com- 

 mon fowl, will readily perceive. With regard to 

 distant or telescopic vision, the power of sight in 

 most (even of the smaller) birds is far superior to that 

 of most of the human race. The writer of this article 

 lias, in his garden, a number of small birds in confine- 

 ment of various genera, and the whole of these he has 



