Sexual Dimorphism 



a bright livery at the breeding season ; this 

 nuptial plumage is by no means invariably con- 

 fined to the cock, so that we are brought face to 

 face with the fact that some hen birds, that are 

 normally inconspicuously coloured, become showy 

 and easy to see at the nesting time, that is to say, 

 precisely at the season when they would seem to 

 be most in need of protection. 



In the great majority of cases of sexual dimor- 

 phism among birds the cock is the more showily 

 coloured. Now, if it be a matter of life-and- 

 death importance to a bird to be protectively 

 coloured, we should expect the showily coloured 

 cock birds to be far less numerous than the 

 dull-plumaged hens, since the former are, ex 

 hypothesi, exposed to far greater danger than 

 the inconspicuous hens. As a matter of fact, 

 cock birds in practically all species appear to be 

 at least as numerous as the hens. Nor can it be 

 said that this is due to their more secretive 

 habits. As a general rule, cock birds show 

 themselves as readily as the hens ; indeed, in the 

 case of the familiar blackbird, the conspicuous 

 cock is less retiring in his habits than the more 

 sombre hen. It may, perhaps, be thought that 

 the greater danger to which the sitting bird is 

 exposed accounts for the fact that hens, not- 

 withstanding their protective colouration, are not 

 more numerous than the cocks. Unfortunately 

 for the supposition, in many sexually dimorphic 



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