6838 Birds. 



slowly, pausing often on upraised wings. The lateral and oblique 

 rays, to the swallow indistinct, to him are clear and accurate. His 

 increased buoyancy enables him to change his direction exactly as he 

 pleases : hence he jerks to one side, takes au insect; jerks .back again, 

 takes another; drops down several feet, stops with a jerk and takes a 

 third; in fine, as the light diminishes, his contortions, zigzags, jerks, 

 swoops, irregular movements of every sort, become more and more 

 violent, till one can scarcely believe they are performed with the per- 

 fect ease to these curious birds which their long continuance and 

 analogy on every side assure us they must be : and as we may define 

 the mode of hunting of the Hirundinidse to be by straight lines of 

 vision or a pencil of rays of great length but small angle, so that of 

 the long-winged Caprimulgida^ is by hemispheres of vision or pencils 

 of rays of much shorter length, but of much greater angle. And this 

 explanation of the peculiar mode of hunting with the piramidigs 

 seems to receive considerable confirmation from similarity of move- 

 ment in the high-flying bats, by which they are so frequently joined 

 — a species, as 1 believe, of Chilonycteris. The bat pursues a direct 

 course, subject to very frequent and violent divergences laterally and 

 downwards. If we wished to represent the waves of sound reaching 

 the ear of the bat from a number of insects among which it was 

 passing, it would be of course by a number of converging lines, 

 exactly as rays of light from the same insects to the eye of a 

 piraraidig. 



" Though during the spring and summer months the appearance of 

 the piramidigs, both in mountain and lowlands, is almost constant, 

 their manoeuvres are by no means always the same. Generally a 

 single bird first appears, uttering its harsh but not unpleasant rattle, 

 takes a long sweep, pauses on raised wings, plunges, and sweeps on 

 again. I have often then remarked that the flight may be represented 

 by a series of rises and falls, the cry always commencing exactly be- 

 fore the bird reaches the turning points. The blowing noise is pro- 

 duced by one of these falls of unusual depth, which I estimate 

 variously from fifty to a hundred and fifty feet, the sound occurring at 

 the curve of recovery, which the bird makes with great rapidity. The 

 original height is then gained by a rise or two higher than the suc- 

 ceeding falls. It is worthy of remark, as showing the difference in 

 buoyancy of the two species, that daring the great descent very con- 

 siderable action of the wings is observable in Chordeiles, and an 

 oscillation amounting to a half or quarter turn of the body in alternate 

 directions, reminding us of 'shooting' rooks, but the same manoeuvre 



