The central question in this paper concerns the mechanism by which displays develop their species-specific sterotyped form. To this end the ontogeny of display behaviour in the black-headed gull was studied in birds kept and raised in aviaries. First it was analyzed whether the complete adult form of the display is present early in the chick, or whether it develops gradually over a longer period. Second, to answer the question whether experience with incomplete forms of display is necessary to develop the complete form, the normal display development was manipulated. The descriptive part of the results deals with a detailed quantitative description of the form development in 3 agonistic displays: choking, oblique and forward, and with the development of the begging display. Data are presented concerning: ontogenetic changes in frequency of several form elements of postures and vocalizations, changes in their combinations, changes in duration of displays and changes in the context and sequences of displays. The development of these displays is summarized. Choking develops from crouching via incomplete forms of choking. First a choking-like vocalization develops in which the notes are more and more placed in a stereotyped rhythm. This call is performed by young chicks first during crouching, but later almost exclusively during choking-like postures in which the bill points downwards. During ontogeny, these bill-down postures are increasingly performed with extended carpal joints, with a tilted body, with head movements, and while standing and not hiding, in this order. From the age of two weeks on, the young regularly extend the neck upwards while choking. These postures are increasingly performed with a particular modification of the choking-like vocalization, which gradually develops to the adult long-call, typical for the adult oblique display. Two independent sound sources in the vocal apparatus of the gull seem to be involved in the development of the long-call. Early oblique-like postures are often performed with the bill held down, as in choking. Older young perform oblique-like postures more often with bill positions typical for adults, but with the neck held vertically. Young older than 10 weeks often perform normal adult oblique postures. By this time the duration of the postures has increased considerably. The sequence of the oblique with the forward, which is typical for adults, is also regularly seen from this time onwards. Begging consists of a rapid up-and-down movement of the head; during every downward movement a specific call is uttered. In very young chicks, this display is hardly distinguishable from the alert posture. During ontogeny, the duration of the pumping bouts increases, while the form changes. This change in form is due to the fact that the incomplete form of the forward becomes integrated in the begging display, while this posture also changes in form. This integration of the alert and the incomplete form of the forward is reflected by the fact that in the begging display both the call typical for the alert posture, and the call typical for the incomplete forward posture, is given. The forward develops by a gradual lowering of the head and a more pronounced extension of the neck, while the carpal joints are increasingly raised. Adult gulls often perform the posture with an upward pointed bill and a bent in the neck. In accord with data from the literature it was found that under special conditions the young chick is capable of performing adult display. As a rule, however, before the tenth week of age, young gulls show incomplete forms of display. All species-specific adult displays, postures as well as vocalizations, develop gradually towards a more complete and pronounced form. This process takes place by addition of new form elements to already existing combinations, and by changes in the form of some elements. A display can also be built upon elements of another display. Nevertheless, the development of the form of the displays cannot be attributed to an increasing amount of coordination of the different elements; only the expression of the complete pattern increases with age. For the occurrence of one form element of an adult display in the precursor display of the chick is positively correlated with the occurrence of other such elements in that incomplete display. This already is the case early in ontogeny, before the complete form is regularly performed. The adult displays are not completely fixed in their form, and the variability in some displays of adults is of the same order as in the chick. The second part of this paper deals with an experiment in which the normal course of the development of display behaviour was manipulated. Since the displays seem to develop during social interactions with parents (the begging display) or with intruders on the territory (the agonistic displays), young birds are raised in small groups with conspecifics of the same age. Agonistic- and begging interactions in these groups are rare an the development of display behaviour, both in frequency and in form, is retarded. Based on the frequency of overt aggression and on the plumage of the birds, it is likely that in these birds also the development of agonistic behaviour and the testosterone production is retarded. At the age of 15 weeks, these birds are confronted with unknown conspecifics. Although the frequency of the displays increases during the test, the form of the postures still appears to be incomplete; the birds do not seem to be able to perform complete display postures at an age at which normally raised birds perform complete display very often. Based on the data of both the descriptive study as well as the experiment, possible mechanisms for the development of display behaviour are discussed. The main items are: "imitation"; "matching" on the basis of feedback of the performed display via reactions of conspecifics or proprioceptive information and development of motivational factors including testosterone. This discussion lays the basis for a series of experiments which will be reported separately.