20 THE AUKS 



ground. As no guillemots had started to lay at the time I made the 

 preceding observations (May 9-13, 1911), I was unable to verify this 

 detail, but the assertion of probability seems to me more than justified 

 by the fact that I saw a razorbill, which species is also in the habit 

 of pushing its egg under its breast, perform the identical nibbling 

 motions before it actually put its bill on the farther side of the egg 

 in order to roll it into place over the side of its foot. 



If this use of the same act for more than one purpose were an 

 isolated fact, it might pass as a mere coincidence. But it is not. 

 Birds, being creatures with limited means of expression, naturally 

 tend to make a familiar act do duty in different ways. The robin's 

 song, for example, expresses love in spring, and in winter is a battle 

 note or a sign of well-being. Further, the act may be primary in one 

 of its uses and derived in the other. For example, the bowing display 

 of the guillemot has all the appearance of being derived from the 

 more primitive act of pushing the egg into place. The jerking of the 

 head in the air that follows the bowing action may be explained in 

 the same way as a development from the nibbling caress, for, as we 

 have seen, the bird often makes nibbling movements without actual 

 contact. In the chapter on Gulls, similar examples will be given. 

 The subject cannot be pursued further here. It leads directly to the 

 very large question of the evolution of animal behaviour as distinct 

 from animal structure. It is one to which field ornithology could 

 make most important contributions. 



The razorbill, besides the billing and the nibbling caress already 

 described, also jerks its head about. I have not seen it indulge in the 

 guillemots' curious bowing movements ; indeed, it has less room for 

 display of any sort, owing to the less open nature of its nesting-place. 

 But it has seemed to me to be more vocal when in its love transports, 

 its chief effort being a harsh continuous croaking note. Generally 

 speaking, however, the guillemot is a much noisier bird, its note being 

 present to the ear almost continuously during the height of the breed- 

 ing season. It is generally described as "murr" and as such has 



