In the present paper are regarded as 'preening', besides the preening movements performed with the bill, also other movements directed to the care of the plumage, such as 'shaking', 'head-shaking' and 'head-rubbing'. Various preening movements are distinguished by the part of the plumage. These various movements are performed in a certain order in the preening bouts 'after bathing'. It is argued that this order indicates differences in threshold value. Head-shaking, breast- and shoulder-preening have low threshold values; shaking(?), preening of the inside of the wing, the tail and the pinions, and head-rubbing have high thresholds; back-, wingbow- and wing-preening have intermediate thresholds. Higher threshold-values go together with greater complexity of motor-pattern. Both threshold values and number of movements per minute, which are positively correlated, are regarded as indicative of the level of the preening drive. This drive fluctuates rapidly and extensively in the first few minutes of the bouts after bathing, more slowly and over a narrower range in later minutes. These are short fluctuations which are superimposed upon one long fluctuation, consisting in a rapid increase, followed by a more gradual decrease. Data concerning brooding Terns indicate inhibitory relations between various behaviour systems: intensive brooding inhibits preening and nest-building strongly, escape rather strongly, and aggression to a certain extent. Escape has an inhibitory influence on brooding and nest-building. It is shown that the occurrence of preening 'during brooding' in most cases is associated with a weak activity of the (otherwise) inhibiting system (brooding), and it therefore is thought to be caused by disinhibition and further by (probably permanently present) activating factors for the preening system. In these cases neither the appearance of the movements, nor the behavioural context provides any reason to regard them as displacement preening. Moreover the composition of this preening (the types of movement and their percentage of the total) is similar to that of preening 'after bathing'. However, other cases of preening are specifically connected with (ambivalent) aggressive or escape behaviour: head-shaking often immediately follows the relatively aggressive 'gackering'; one or a few preening movements often are performed immediately, or sometimes a few (2-3) minutes after escape behaviour. In these cases the preening has to be considered as displacement, because of the (sometimes) "frantic" appearance, and mainly because of the context. Moreover, its composition differs from that of preening 'after bathing', in that low threshold movements are especially frequent. Several data indicate the displacement character of the preening which occurs 2-3 minutes after disappearance of overt (escape) behaviour. It is especially stressed that a movement may still be a displacement, though it is not directly accompanied by overt escape behaviour, and even when no such behaviour occurs at al. The latter is born out by the case of the non-aggressive opponents of 'gackering' birds, which do not perform any escape/aggressive behaviour, but still show head-shaking. It is shown that the displacement preening results from conflicts, either between brooding and escape ('nest-reliefs', 'nest- and 'air-alarms'), or between aggression and escape ('gackering'). The occurrence of displacement is shown to depend on the relation between the conflicting drives: the strengths of these must not diverge too much from a certain ratio, which is called 'effective equality'. On the basis of the established inhibitory relations between the various behaviour systems a hypothesis on the displacement mechanism is offered ('disinhibition hypothesis'). The idea of "sparking over of motivational impulses" is rejected. It is argued that a conflict-which occurs when the conflicting drives are 'effectively equal'-consists in mutual inhibition of these two drives, resulting in removal (reduction) of their inhibitory activity on other systems. A conflict between brooding (aggression) and escape thus results in removal (reduction) of the inhibition by brooding (aggression) on preening. Provided some '+factors' for preening are present, preening will then occur.. From this point of view the terms "allochthonous" and "autochthonous" are superfluous. The frequency and intensity of displacement (the latter measured by the number of movements per case and their threshold values) are as a rule positively correlated with the strength of both conflicting drives. It is argued that this is due to a greater average degree of 'effective equality' of strong conflicting drives, since the degree of 'effective equality' is thought to determine the chance, the degree and the duration of disinhibition of the displacement system. When both drives are strong, but one strongly exceeds the other ('inequality'), the chance of displacement decreases considerably, but if it does occur it is intense. So, in these cases-and some other-frequency and intensity of displacement are not correlated. Rain is shown to have a preening-releasing effect in all situations considered. Head-shaking seems to be facilitated most strongly. The influence of rain is likely to be a direct activation of the preening system. The increase of the preening frequency in rain (of a constant strength) is not equal in the various situations : the 'degree of facilitation' is correlated with the frequency of preening during dry weather and further it depends on the type of situation (conflict or non-conflict situation; type of conflict). For this an explanation is given on the basis of the view on the displacement mechanism and the nature of the conflict presented in this paper.