On the ontogeny of display behaviour in the black-headed gull ( Larus ridibundus )

In a study of the form development of the species-specific displays in the black-headed gull, a gradual change of these motor patterns was found, as well as a retardation of this change, in birds raised in a situation lacking the normal social context (GROOTHUIS, 1989). In this experimental context, low frequencies of aggressive and fear behaviour were also found. These results have led to the hypothesis that the ontogenetic change of display behaviour is influenced by the development of internal factors for agonistic behaviour. To put this idea to a test, a study was started to investigate the relation between the development of aggressive and fear behaviour and of display behaviour during ontogeny. Standard stimulus experiments, simulating an adult intruder on the territory, were carried out at least once a week during the first 9 weeks of age in groups of territorial, hand-raised birds. The frequencies of two forms of fear behaviour, hiding and escape, and of aggressive pecking gradually increased in this order. The proportion of aggressive behaviour that was immediately followed by withdrawal was high in 2-week old chicks, but decreased considerably with age. The frequency of sudden stops after approach and of incomplete aggressive pecks increased with age, suggesting that overt aggression became more inhibited by fear, resulting in longer lasting interactions with more display behaviour. This interpretation is supported by the finding that stopping as well as sitting occurred in the behavioral chain at the switch between overt aggression and overt escape. The displays gradually changed in frequency and in form, via incomplete ones to the adult complete form. Oblique-, forward- and choking-like postures were almost exclusively performed in an agonistic context, in contrast to the begging display. It is concluded that each of these three agonistic displays develop from postures, dominated by a tendency to escape or to hide, to complete display by an addition of a simultaneously activated tendency to behave aggressively. This is based on: 1. similarities in frequency changes during ontogeny of overt aggressive and fear behaviour on the one hand, and the displays on the other hand; 2. the occurrence of the different displays during sudden stops after aggressive approach, during withdrawal, and in sitting which followed escape and was often followed by aggression; 3. the temporal relationship of the displays with overt aggression and with each other, and differences in this respect between incomplete and complete display; 4. similarity in form of the displays with overt aggressive and fear behaviour. Functional aspects of this development are discussed. The motivation of display in older gulls is studied separately, mainly by analysing normal social interactions. The increase in display frequency and decrease in overt aggression and fear during early ontogeny was maintained in later ontogeny. As a consequence, adults hardly showed overt aggressive behaviour. This finding could be explained by assuming an increasing mutual inhibition of each other's overt expression by internal mechanisms controling aggression and fear. However, this explanation could not account for other findings: 1. in contrast to young birds, adults frequently perform display outside agonistic contexts; 2. in contrast to young, adult birds no longer perform their display during those categories of locomotion for which evidence was obtained that they were caused by a simultaneous activation of aggression and fear; 3. adult gulls perform the oblique-forward sequence simultaneously with those of the opponent. The increase in synchronously performed display during late ontogeny suggests an increasing independence of display of the precise motivational state of the animal. It is concluded that the displays, once developed under the influence of internal factors controlling aggressive and fear behaviour, become fixed in form and independent of these factors. It seems as if adult birds may use these motor patterns as a kind of subroutines, applicable in different behavioural programs. If aggression is performed by adults, display is almost always performed also, suggesting that internal factors for agonistic behaviour, when aroused, still control display behaviour. It is suggested therefore that the ontogenetic change in causal organisation of display behaviour in gulls is more adequately described by the term "extension" of motivational factors than by the term "emancipation" of the original ones.