Display of Undecorated Cocks
Finn observed that a young hen Bird of Paradise (Paradisea apoda) in the London Zoological Gardens, mated with a fully adult cock in the next compartment although a young cock in female plumage in her own compartment did his best to show off.
It would thus seem that the very limited evidence at present available is not sufficient to sustain the theory that the hens select the most attractive of their suitors. It is significant that plainly-coloured species of birds show off with as much care as their gaily-plumaged brethren; and, if they be nearly allied, assume similar courting attitudes. Thus the homely-attired males of the Spotted-bill (Anas poecilorhyncha), Gadwall, and Black Duck (Anas superciliosa), show off in precisely the same way as does the handsome mallard.
Howard describes and figures in his excellent and beautifully illustrated monograph the elaborate display at the pairing season of some of our plain-coloured little warblers. The skylark has also a notable display.
The common partridge assumes a nuptial attitude similar to that of the pheasant, and, although the cock of the former species has nothing brilliant to show off, the hen partridge pays far more attention to the display of her suitor than does the hen pheasant.
The fact that some cock birds show off after the act of pairing seems to tell against the theory of sexual selection, or at any rate to indicate the purely mechanical nature of the performance. Finn has witnessed this post-nuptial display at the Zoological Gardens (London) in the pied wagtail, the peacock, the Andaman Teal (Nettium albigulare), the Avocet, the Egyptian Goose (Chenatopex ægyptiaca), and the Maned Goose (Chenonetta jubata).
Another objection to the theory that the bright colours of cock birds are due to feminine selection is presented by those birds which breed in immature plumage. Darwin admits that this objection would be a valid one “if the younger and less ornamental males were as successful in winning females and propagating their kind as the older and more beautiful males. But,” he continues, “we have no reason to suppose that this is the case.”
Unfortunately for the theory of sexual selection, there is evidence to show that the cock Paradise Fly-catcher (Terpsiphone paradisi) in immature plumage is quite as successful in obtaining a mate as is the cock in his final plumage. The cock of this beautiful species has a chestnut plumage in his second year, and a white one in the third and subsequent years of his life. Nevertheless, a considerable proportion of the nests found belong to chestnut cocks.
Plumage of Herons
Darwin was of opinion that any novelty in colouring in the male is admired by the female; and in this manner he sought to overcome some difficulties to his theory which certain birds presented.