Birds. 



6759 



announce to the host of birds below the direction of its movements, 

 and thus combine the movements of the whole flock. And we may 

 observe that by the introduction here of the ordinary instinct of 

 swifts to scream when they pursue each other, the hunting birds can 

 concentrate their powers of vision on their search, and by ear know 

 the direction those searches must take. If, therefore, every bird in 

 the flock regulates its course by the slow movements of the column, 

 which thus gives them ample time to explore thoroughly, it will be 

 obvious that, whatever the movements of each individual bird, quite 

 unconsciously the column will always tend to be over the centre; and 

 we might expect exactly what I observed on the 3rd of September, 

 that the hunting birds were thickest immediately round it. 



" But though now it can be comprehended how these large flocks of 

 birds keep together in their search, another most formidable difficulty 

 presents itself. Supposing a couple of birds (as I observed on the 20th 

 of August), sweeping up to a troop of Progne, find they are feeding on 

 wasps — a prey which will suit them : the successful birds do not 

 hesitate a moment ; their wild, irregular dashes are suddenly changed 

 to a dart in a straight line, a rapid whirl and a dart back again ; in 

 fact, they begin at once to feed. How then is the column to know it, 

 or be prevented from moving off in another direction ? This, I believe, 

 is effected by a most remarkable structural change, involving a cor- 

 responding habit. To describe this change of structure I use the 

 words of your exact old Doctor, * Eyes large, deep sunk in the head, 

 with remarkably large eyebrows,' only remarking that the surface of 

 the feathers is considered by him as synonymous with ' head.\ If the 

 skin and feathers are removed the eyes are remarkably prominent. 

 He afterwards adds, ' the head large, like whip-poor-will.' I will add 

 to this that the pupil is very large, lustrous and of a very deep blue. 

 So little is, I believe, known as to the effect upon vision which the 

 almost endless changes of form we meet with have in this organ, 

 that we could not say that such and such a form is unnecessary in a 

 particular case ; but, reasoning from analogy, we have examples of 

 other birds which pursue insect prey precisely, so far as we can see, in 

 the same manner as these birds : the humming-birds are perhaps even 

 more rapid in their movements, yet none of these are remarkable for 

 large lustrous eyes ; quite the contrary, they are rather apt to be 

 smaller than larger than those common to the greatest number of birds. 

 But if we learned that a particular species had such wants that it was 

 necessary it should be able to take a careful survey of objects below 

 it whilst it floated above, we should be able to say, with perfect cer- 



