64 



Meeting held in the Library of the Philosophical Hall, October 27th, 

 1890, James W. Addyraan, B.A., in the chair. 



SOME OF THE POSSIBLE CAUSES OF VARIATION IN THE SHAPE AND 

 COLOUR OF THE EGGS OF BIRDS.* 



H. BENDELACK HEWETSON, M.R.C.S., F.L.S. 



In the consideration of a subject of this sort we see what a wonderful 

 tendency there is in nature to adopt methods of self-preservation and 

 defence in one way or another. We are dealing with a vast area of very 

 deep truths which cannot be overlooked. It involves not merely the 

 study of birds' eggs, but also of the laws of evolution, so ably put before 

 us by Darwin and Wallace. It is only by the observation of apparently 

 infinitesimal causes that we find there is a great work going on, of the 

 fringe only of which we have knowledge. Take, first of all, the case of 

 the Bunting. What simple means for concealing the eggs the bird adopts, 

 viz., that of sitting on them. Otherwise they would be pretty evident. 

 Not only does it conceal the eggs by overhanging herbage, as in the case 

 of the Lai-ks and Tit-larks, but, in addition, there is a great concealing 

 power in the colour of the bird itself. Perhaps it would be well to 

 consider, first of all, how eggs are formed so as to imitate their 

 surroundings. Mimicry is not a good word with which to describe the 

 process, for it seems to be associated with something frivolous. In 

 Nature there is no ridiculous side, and to the general public the word 

 mimicry, applied to the habits of birds, is misleading. Imitation is a 

 better word than mimicry, because there is no second meaning behind it. 

 It is a wonderful thing, then, that a bird, by a process of evolution, 

 should so select its surroundings before depositing an egg that they 

 should afford protection to that egg. There is not a better instance of 

 this than the Kentish Plover, which literally selects a stone exactly like 

 the egg it lays, and deposits the egg near the stone. Professor Newton, 

 when lecturing to his students at Cambridge, one day held up before them 

 an egg of this bird and a stone found beside it, and they could not tell 

 which was which. The probability is that the stone is selected before the 

 egg is laid. Again, the Eing Dotterel selects exactly the kind of nest 

 which will best prevent detection. It does this with an absolute 

 knowledge that the colour of the eggs is an imitation of the surroundings. 

 Supposing the rising of unexpected tides compels the Dotterel to retreat 

 from the sea-shoi-e on to ploughed land, it alters its habits in a moment. 

 It knows ]3erfectly well that, surrounded by the brown soil its eggs will 

 be easily seen, and so it collects shells off the beach and places them round 

 the nest so as to detract attention from the eggs. Mr. Hewetson said he 

 had frequently seen it done. The power of imitation is strongly noticeable 

 in the ease of the Lesser Tern, one of our most graceful birds, and which is 



* A paper read before Section D of the British Association meeting, 1890. 



