In the case of birds, however, feminine preference comes more into play. It is well known that cocks display their charms to the hens at the breeding season, and Darwin believed that the hen selected the most beautiful of her rival suitors.
“Just as man,” he writes (p. 326 of The Descent of Man, new edition, 1901), “can give beauty, according to his standard of taste, to his male poultry, or, more strictly, can modify the beauty originally acquired by the parent species, can give to the Sebright bantam a new and elegant plumage, an erect and peculiar carriage, so it appears that female birds in a state of nature have, by a long selection of the more attractive males, added to their beauty or other attractive qualities.”
Thus the theory of sexual selection is based on three assumptions. Firstly, that there is in all species competition among the males for females with which to mate. Secondly, that this results in either “the law of battle” among the males, or selection by the female of one among several admirers. Thirdly, that the female selects, as a rule, the most attractive of her suitors.
The evidence upon which Darwin founds this theory may be thus summarised:—
1. In cases where the sexes differ in appearance, or power of song, it is almost invariably the cock who is the more beautiful or the better singer, as the case may be.
2. All male birds that possess accessory plumes or other attractions, make a most elaborate display of these before the females at the mating season, hence “it is obviously probable that these appreciate the beauty of their suitors.”
3. Darwin was able to cite specific instances in which the hens showed preference.
In the case of polygamous species there can be no doubt that there is considerable competition among males for their wives. It cannot be said that the contention is so well established in the case of monogamous species. D. Dewar suggests that circumstances may occur in which the hens have to fight for the cock, or in which the male is in the happy position of being able to select his mate. He states his belief that in many cases the selection is mutual, as in the case of human beings.
“I have seen,” he writes, on page 13 of Birds of the Plains, “one hen Paradise Flycatcher (Terpsiphone paradisi) drive away another and then go and make up to a cock bird. Similarly, I have seen two hen orioles behave in a very unladylike manner to one another all because they both had designs on the same cock. He sat and looked on from a distance at the contest.”
Darwin quotes, on page 500 of The Descent of Man, a case of a male exercising selection: “It appears to be rare when the male refuses any particular female, but Mr Wright of Geldersley House, a great breeder of dogs, informs me that he has known some instances: he cites the case of one of his own deerhounds who would not take any notice of a particular female mastiff, so that another deerhound had to be employed.”