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Ambivalence in the Structure of Display Vocalizations of Gulls and Terns: New Evidence in Favour of Tinbergen's Conflict Hypothesis?

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This paper deals with the question whether and how motivational systems for attack and escape may find expression in social displays of gulls, with special reference to vocalizations. A comparison of sonagrams of the vocal display 'Long Call' of common tern, arctic tern and little gull with calls made by these species during overt attack and overt escape behaviour shows that attack and escape elements can be recognized in the structure of the Long Call (Figs 2 and 5). The alternation of notes of long and short duration in the Long Call of the little gull (Fig. 3) is regarded as an example of successive ambivalence; long and short notes representing attack and escape tendencies respectively. Whether nowadays occurrence of the Long Call is still under the influence of motivational systems for attack and escape has been studied by analysing the extent to which elements of attack recognized in the Long Call of the little gull alter with presumed motivational changes. It turned out that note duration increased with increase of the incidence of overt attack in three different situations. Similar changes were observed in form elements of the Oblique Posture which typically accompanies the Long Call by birds on the ground (Fig. 6). It is argued that it seems to be the rule rather than exception that the same gull display (visual as well as vocal) occurs in different contexts in which the probability that it is followed by attack or escape varies greatly. Evidence available from studies of a great number of gull species suggests that motivational state underlying a particular display may vary considerably and that different motivational states are reflected by the extent to which motivation specific components of the display manifest themselves. Thus, it is not the display itself (as proposed by TINBERGEN, 1959 and MOYNIHAN, 1955a), but certain elements of which it is composed, which are under the control of the attack and escape motivation. BEER (1975) has experimentally shown, that feature differences within Long Calls of laughing gulls determine different reactions. In case of the little gull, feature differences communicating about motivation might well explain the different reactions to Long Calls in different circumstances.

Affiliations: 1: Zoological Laboratory, University of Groningen, P.O. Box 14, 9750 AA Haren (Gr.), The Netherlands


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