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This female preference for male ornaments can actually be a threat to the survival of the males. The scarlet-tufted malachite sunbird is an iridescent green bird that lives high on the slopes of Mount Kenya where it feeds on the nectar of flowers and on insects that it catches on the wing. The male has two long tail streamers, and females prefer the males with the longest streamers. By lengthening the tail streamers of some males, shortening those of others, adding weight to those of a third group and merely adding rings of similar weight to the legs of a fourth, two scientists were able to prove that female-preferred tail streamers are a burden to their bearers. The ones with lengthened or weighted tails were worse at catching insects; the ones with shortened tails were better; the ones with only rings on their legs were as good as normal.18

Females choose; their choosiness is inherited; they prefer exaggerated ornaments; exaggerated ornaments are a burden to males. That much is now uncontroversial. Thus far, Darwin was right.

Despotic Fashions

The question Darwin failed to answer was: why? Why on earth should females prefer gaudiness in males? Even if the ‘preference’ was entirely unconscious, and was merely an instinctive response to the superior seduction technique of gaudy males, it was the evolution of the female preference, not the male trait, that was hard to explain.

Some time during the 1970s, it began to dawn on people that a perfectly good answer to the question had been available since 1930. Sir Ronald Fisher had suggested then that females need no better reason for preferring long tails than that other females also prefer long tails. At first, such logic sounds suspiciously circular, but that is its beauty. Once most females are choosing to mate with some males rather than others and are using tail length as the criterion – a big once, granted, but we’ll return to that – then any female who bucks the trend and chooses a short-tailed male will have short-tailed sons. (This presumes that the sons inherit their father’s short tail.) Yet all the other females are looking for long-tailed males, so those short-tailed sons will not have much success. At this point, choosing long-tailed males need be no more than an arbitrary fashion; it is still despotic. Each peahen is on a treadmill and dare not jump off lest she condemn her sons to celibacy. The result is that the females’ arbitrary preferences have saddled the males of their species with ever more grotesque encumbrances. Even when those encumbrances themselves threaten the life of the male, the process can continue – so long as the threat to his life is smaller than the enhancement of his breeding success. In Fisher’s words, ‘The two characteristics affected by such a process, namely plumage development in the male and sexual preference in the female, must thus advance together, and so long as the process is unchecked by severe counterselection, will advance with ever-increasing speed.’19

Polygamy, incidentally, is not essential to the argument. Darwin noticed that some monogamous birds have very colourful males: mallard, for example, or blackbirds. He suggested that it would still pay males to be seductive and so to win the first females that are ready to breed, if not the most, and his conjecture has largely been borne out by recent studies. Early nesting females rear more young than late nesting ones and the most vigorous songster or gaudiest dandy tends to catch the early female. In those monogamous species in which both males and females are colourful (such as parrots, puffins and peewits) there seems to be a mutual sexual selection at work: males following a fashion for picking gaudy females and vice versa.20

Notice, though, that in the monogamous case the male is choosing as well as seducing. A male tern will present his intended with fish, both to feed her and to prove that he can fish well enough to feed her babies. If he is choosing the earliest female to arrive, and she is choosing