SEXUAL COLOUR-DIMORPHISM IN BIRDS. 35 



In some cuckoos, then, it would seem that sexual dimorphism is due, 

 neither to sexual selection, nor to excess of vitality on the part of the 

 males, but is rather correlated in some unknown way with the gen- 

 erative organs. 



The sexual dimorphism of the koel is possibly to be explained in a 

 different manner. It will be remembered that these birds victimise 

 crows. Crows seem to be aware that the koel is an enemy, for they 

 never lose an opportunity of attacking the male bird. But, unless my 

 memory serves me false, I have never seen a crow chase a female koel. 

 Is it a fact that it is only the male koel which is detested by the crows ? 

 Do these latter fail to recognise the female as one of the same species ? 

 If so, the sexual dimorphism in this case is easy to account for. It is 

 important that the male should be conspicuous in order to attract the 

 attention of the crows and lure them away, while the female cuckoo 

 deposits her egg. Young koels, when first they acquire feathers are 

 black, presumably in order that their foster parents may mistake them 

 for young crows. Later they assume the mottled plumage of the 

 adult female, presumably in order that the crows shall not recognise 

 them as their arch-enemy, hence the wonderful immunity from attack 

 which young koels seem to enjoy. 



In the case of cuckoos which victimise small, helpless birds such 

 dimorphism is unnecessary for the welfare of the species, and so must, 

 I think, be attributed to the influence of the sexual organs. 



This brings me to a point which I would venture to emphasise, 

 viz., that not one but many causes have operated to cause external 

 sexual dimorphism. The sexual dimorphism of one species is to be 

 accounted for in one way, and that of another in an altogether different 

 manner. 



It is probable that that form of sexual selection whereby the most 

 pugnacious and mettlesome males secure the most wives has contributed 

 largely to the sexual dimorphism of polygamous species, in which the 

 dimorphism is often so very marked. But the fact that the paradise 

 flycatcher is monogamous shows that it is possible to ascribe too much 

 importance to this factor. 



Then, again, it is tolerably certain that in the case of birds sitting on 

 open nests, natural selection has tended to keep the general hue of 

 female birds dull and inconspicuous, 



