6760 



Birds. 



tainty, it must have a prominent, lustrous eye with a large pupil (deep 

 blue often), carefully screened from the light above by a prominent 

 eyebrow, that it may be wholly unembarrassed in its watch over objects 

 below. Now let us consider the effect of the central position of the 

 column, its commanding height, if suddenly, in any one or two birds 

 at any part of the vast circle it is over, the motions change, like the 

 two birds who had reached the little flock of Progne, detailed above. 

 Can we suppose that an alteration of action, observable to us at a con- 

 siderable distance, for a moment escapes the keen scrutiny of a hundred 

 telescopic eyes watchfully gyrating above ? We may be quite sure it 

 does not, for the sudden descent of the column may often be witnessed. 

 Now it will be understood what will be the effect of this on the 

 scattered birds who are less successful : the column, which has been 

 the centre of their movements, is gone ; the birds then rush up in the 

 direction it has disappeared, or, rising above intervening objects, catch 

 sight of it busy feeding. And thus we have the extraordinary spectacle 

 of dozens of birds dashing to the very spot where the prey has been 

 found, as if each had received accurate and separate intelligence. 

 Then a boundary is traced out by a flickering succession of rapid 

 wheels — all within is a storm of dashing courses in every direction at 

 once. The moment the disturbed or angry insects rise from the 

 plants or grass their fate is certain. Others dart out and probably 

 take the wasps as they return to the spot. If we multiply 200 birds 

 by 30 wasps' heads I found to be the allowance of one in an hour and 

 a half, we have 6000 of these formidable insects destroyed in that 

 short time, but I think the number was probably far more. And then, 

 the work accomplished, this wonderful army resumes its aerial march, 

 the magnificent living standard re-erects itself, and its screams growing 

 faint and still fainter, recall the scattered rear. 



" I would guard against an impression which may possibly arise 

 from the foregoing details ; namely, that in their curious habits there 

 is anything like the rigid instincts observable in other classes of 

 animals. I believe the whole to be carried on by the very simplest 

 instincts, more or less observable in most Hirundinidoe. The same 

 instinct which makes the searching birds join Progne, and often 

 Progne join them, induces them to keep near the column, the column 

 to descend when it sees the prey is found, and the rest to rush after 

 the missing column. It is but a very extraordinary modification of a 

 well-known habit: it results in giving the column a central position, 

 precisely the one where it can best overlook the whole ; its motion is 

 probably directed by the direction it observes in the search of a 



