1-14 Ml'. C. F. M. Svvynnerton on Rejections [Ibis, 



that necessities of diet have much to do with the early- 

 stage now represented by the relations between the Cow- 

 birds Molothrus badius and M. rufo-axillaris, though it is in 

 any case only natural that the first layers in nests of other 

 species should go, where it is available, to a species nearly 

 related to their own. 



Again, when the next and bigger step was taken of placing 

 eggs in tlie nests of quite unrelated birds, food will have 

 remained a prime consideration, and this seems to me to 

 have a bearing on another point. For, surely, the safest 

 rule in this connection, as well as the natural thing to 

 happen, would be for the Cuckoo to base its choice primarily 

 on recognition of the foster-parents that had successfully 

 reared itself. It is quite true that in butterflies, in which 

 recognition is primarily by smell, a male will, after a first 

 pairing, recognize also by sight, as is evidenced by the 

 courtship of model by mimic and mimic by model that 

 I have myself often witnessed. It is similarly possible 

 that, having seen its own egg, a Cuckoo may be influenced 

 by egg-coloration in its choice of nests ; but I cannot help 

 feeling that the order of probability, or, if (as is sometimes 

 likely) all three means of recognition are used, the order of 

 importance will be, (1) appearance of foster-parents, (2) of 

 nest, (3) of eggs. The criticism, frankly adduced by 

 Major Meiklejohn himself, that Cuckoos regularly deposit 

 in Hedge- Sparrows' nests eggs unlike those of the foster- 

 parent, seems to me to tell too strongly against the opposite 

 view to be lightly passed over. 



Selection would soon follow the adoption of the habit of 

 placing the eggs in other birds' nests — selection of dis- 

 crimination in the more usual hosts and of deceptive 

 coloration in the Cuckoo's egg. Discrimination may be 

 rarer, and mimicry less needful, at first than later, and it is 

 in this connection that the transition so well illustrated 

 by my experiments is suggestive : the transition between 

 such a bird as the Hedge-Sparrow must be (my experiments 

 on any one species were insufficient to convince me that I 



