The acquisition of mates does not depend entirely upon strength and victory in battle. Many male mammals have crests and tufts of hair, and other marks of beauty, such as bright colouring, are often conspicuous. These are used to attract the females. The incense of odoriferous glands, which become specially functional during the breeding season, are another frequent means of sexual attraction.[56] Even many of the amatory duels are not really fights between rivals. They are rather parades, or tournaments, used by the males as a means of displaying their beauty and valour to the females. This is frequent among the contests of birds, as, for instance, the grouse of Florida (_Tetras cuspido_), which are said to assemble at night to fight until morning with measured grace, and then to separate, having first exchanged formal courtesies.[57] It is among birds that the notes of joy in love break out with a wonderful fascination. They are the most perfect of lovers; strength is often quite set aside, and the eye and ear of the mate alone is appealed to. The males (and also, in some cases, the females) use many aesthetic appeals to stimulate passion, such as dancing, beauty of plumage, and the art of showing it, as well as sweetness of song and diverse love-calls. There are numerous examples of affectionate partnerships between the sexes, in some cases lasting for life. The female Illinois parrot, for instance, rarely survives the death of her mate. Similarly the death of either sex of the _panurus_ is said to be fatal to its companion. The affection of these birds is strong; they always perch side by side, and when they fall asleep one of them, usually the male, covers the other with its wing. The couples of the golden woodpeckers and doves live in perfect unison. Brehm records the case of a male woodpecker who, after the death of his mate, tapped day and night with his beak to recall the absent one, and when at last discouraged, he became silent and never recovered his gaiety.[58] According to some estimates monogamy prevails among ninety per cent of birds.[59] This is explained by the steady co-operation of both sexes in forming the home and caring for the young, for it is surely the working together which causes their love to outlast the excitement of the procreative season. Sometimes we find this affection flowing out into a wider altruism, extending beyond the family to the social group; which again is surely at once the condition and result of these beautiful and practical love-partnerships.