SUMMARYThe Ptilonorhynchidae (bower‐birds) and Ailuroedidae (cat‐birds) are families widespread throughout Australia and New Guinea and endemic to these regions. Only bower‐birds make bowers. The bower is on the ground and has no direct connexion with the nest, which is built in a tree that may be some hundreds of yards away. At each bower is a display‐ground which the owner strews with coloured flowers, fruit and shells, or, in some species, pallid or reflecting articles. These, contrary to general belief, are selected with great discrimination. Members of three genera paint their bowers with fruit‐pulp, or with charcoal or comminuted dry grass mixed with saliva. Cat‐birds, on the other hand, do not erect a special structure.Pioneer nineteenth‐century naturalists, who knew little about bower‐birds, ascribed a courtship function to the above activities and postulated an aesthetic sense as well. During the last three decades, however, there has arisen a school of Australian naturalists, which, though admitting that the displays of bower‐birds may have had their origins in sexual utilitarianism, believe that this has become a minor factor. They hold that modern bower‐birds are intelligent, consciously aesthetic, and that they carry out their complex display activities essentially for relaxation or recreation. This theory is here rejected. Experimental work has shown that gonadectomy inhibits display and bower‐building and that injections of testosterone re‐establishes it. Post‐nuptial (out‐of‐season) display occurs after the seasonal metamorphosis of the testes and the rehabilitation of the interstitium, and is of the same nature as the autumnal display of other birds.Excluding one monospecific genus(Archboldia) of whose bower only fragmentary knowledge is available, true bower‐birds appear to fall into two distinct groups. These are (i) the avenue‐builders(Ptilonorhynchus, Chlamydera, Sericulus) (Fig. i) and (2) the maypole‐builders(Prionodura, Amblyornis) (Fig. 2).Theavenue‐buildersare widespread throughout both Australia and New Guinea. Each lays down a foundation platform of twigs, and in this wedges twin parallel walls of twigs which sometimes arch overhead. At one, or sometimes both, ends of the bower the male birds arrange their display‐things. One species replaces the terminal display‐grounds with two additional walls which are built at right angles to the original ones. It thus achieves three avenues, in all of which it places decorations.Among avenue‐builders the male builds a bower early in the season, after which it attracts a female to the bower and there displays to it at the focal point of its territory. The male rapidly achieves spermatogenesis but the female cycle lags behind. The male displays (‘dances’) for several months, while the female remains passively in waiting. During this lengthy period of androgen liberation there occurs the remarkable display of bower‐painting, vocal mimicry of other birds, and the insatiable collection of coloured or otherwise distinctive display‐things. In one, and possibly several, species the collected display‐objects are coloured similarly to the watching female. With these in his beak the male displays energetically and often violently but does not approach the female. This has led to published references to ‘sexual fetishism’ in bower‐birds. There is, however, a fundamental difference between the above normal and constant avian activity and a form of psychopathic sexual behaviour in man. Likewise, although the male bower‐birds' prolonged preoccupation with display‐objects while he is in a breeding condition involves a form of sexual sublimation, it is important to remember that this too is normal and constant and a means towards, rather than a sublimation of, reproductive efficiency. The display with the ornaments before the watching female continues unabated until the forest becomes seasonally full of the special protein food on which the young are fed. Then only is the male's physical attention transferred from his display‐things to the waiting female. The female now leaves the bower, alone to build the nest and to rear the young. The male continues his display. His testes, unlike those of most birds, do not metamorphose after the young appear. Thus he remains potentially polygamous, although polygamy has not been proved.In ascribing an essentially utilitarian function to the bower and associated activities, it is not suggested that bower‐birds do not enjoy their extraordinary performances. Further, it is emphasized that bower‐birds appear to have developed their display beyond the bounds of strict utilitarianism, since some individuals successfully reproduce without painting their bowers, and it has been experimentally shown that reproduction (in the aviary without competition) can take place in the absence of coloured decorations.Themaypole‐builders,with one exception, are confined to the dense rain‐forests of New Guinea. The fundamental maypole structures are a central growing sapling with a cone of fabric packed around its base. One species, measuring only 9I inches long, extends this cone to a height of about nine feet. Another species surmounts a dwarf cone with a conical, waterproof hut. A third builds over the cone a hemispherical hut fronted by a stockade, and a fourth surmounts the basal cone with a cylinder of moss and fibre which extends up the basic sapling. Fruits, flowers, beetles' wing‐cases and other coloured objects are used to decorate various parts of the structures or their surroundings.Bower‐building of both avenue and maypole types probably originated as a form of displacement activity. The males as well as the females of most passerine birds possess an inherent urge to build nests. To‐day only female bower‐birds seem to build nests. These are built of twigs. The males, on the other hand, have taken to building twig bowers on the display‐ground. Here they spend much of their time during the sexual season. The display‐ground is the focal point of their activity, and, together with the female, of their interest. Nest‐building among females is essentially controlled by the seasonal liberation of hormones. The same appears to be true of male bower‐building. In short, nest‐building is of a bisexual nature, and this has made possible bower‐construction as a displaced building drive, the new product of which has become valuable, ritualized and permanent in the course of the evolution of each species.Bower‐painting is a displaced form of the courtship feeding that is widespread among passerine and other birds. Instead of projecting this attention on the watching female the male does so on his bower which, along with its decorations, is his chief physical preoccupation (apart from feeding and threat) during the weeks or months of spermatogenesis.Of the threecat‐birds,two greenish species indulge in arboreal displays in much the same manner as other passerine birds, whilst the third, the brown stagemaker of tropical Queensland, has taken partly to the rain‐forest floor. Here it clears a space on the ground and covers this with fresh leaves which it cuts each morning by means of a ‘toothed’ beak. The leaves are placed with paler under‐surfaces uppermost, thus creating the more striking effect against the dark earth. From a singing‐stick above the decorated stage the bird calls loudly and continuously and thus advertises its presence and its decorated stage to every potential mate or rival within many hundreds of yards. The precise function of the stage is unknown, but the scanty available data suggest that it, and the vocal display from above, are means toward the cementing of the pair‐bond and effecting the synchronization of the joint sexual processes of the pair. Finally, perhaps, it is the mating place when the monsoon begins and brings with it the seasonal harvest of insect food on which the young will be fed.It has been suggested by cabinet‐workers that ‘bower‐building has been developed coincidentally or perhaps independently’ (Iredale, 1950) and