At all stages of the preparation of this paper I have been given the benefit of Sir Julian Huxley’s very effective help and guidance. In addition, many biologists have advised on various points, notably Professor Ernst Mayr, Dr A. J. Hogan-Warburg, Dr J. P. Kruijt, and Dr H. Lind. The discussion is restricted to birds in which communal and social displays are comparatively well studied and constitute unique examples of extreme ritualization. We can see various stages of their evolution. Following the pioneer work of Selous (1901, notably pp. 68- 95), and especially that of Huxley (1914, 1923), evidence has accumulated to indicate that threat and courtship display—with which we are here concerned—derive originally from movements and postures which are the expression of conflict situations. Ritualization then takes place under the influence of natural selection towards realizing greater adaptive (biological) value of the expressive actions, both as a means of unambiguous communication and as ‘releasers’ of appropriate conspecific behaviour (for more detail see contributions by Huxley, Lorenz, Hinde, and Cullen, in this volume). In the case of ritualized leks and other forms of social display, there is to begin with, a general stimulative function which is also found in individual courtship display. But this effect is greatly exaggerated when several individuals display together. In the case of social displays the stimulation may also promote other activities than reproduction, for example, migration. The transmittance of instinctual ‘moods’ by this means may have pronounced physiological effects and tends to synchronize the activities in question (‘Fraser Darling effect’ with respect to breeding activity; cf., for example, Crook 1965, p. 190). Secondly, as in other ritualized displays, the signalling function is perfected, which means greater formalization and uniformity of motor behaviour, together with exaggeration of the conspicuous structures and colours involved. A third, very important point—well illustrated here—is that ritualization of hostile behaviour reduces actual combat with its ensuing possible damage to the species. Finally, these displays demonstrate the decisive influence of the ecological background and general mode of life of the species in question. Thus, a prerequisite for the specialized communal displays by males for mating purposes—arena displays—is the male’s abandonment of parental duties.
