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1) Great skua (Stercorarius skua) interactions were studied in the club areas of colonies on Hoy and Fair Isle. Attention was centred on reactor response to postures given by actors which did not attack or escape after displaying. 2) The regression of the reactor's escape probability on distance from the actor was analysed for Facing and Not Facing orientations of the actor. 3) Responses shown to the postures Oblique/Long Call/Wing-raising (Facing and Not Facing), Neck Straight/Bill Straight (Not Facing) and Neck Straight/Bill Straight/Long Call (Not Facing) were similar, and depended on distance, rather than the posture. This rules out the possibility that these postures constitute distinct threat signals. However, Neck Straight/Bill Straight (and Neck Straight/Bill Straight/Long Call) Facing showed a distinct pattern of response which indicated that they formed a (single) threat signal. 4) This pattern was consistent across the different colony-year samples. 5) It was concluded that the great skua behaviour patterns analysed here acted to define different situations rather than providing information about signaller intentions.

In: Behaviour
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Abstract

1. Great skuas Stercorarius skua use a range of displays in agonistic interactions in the club areas of their breeding colonies. We examine whether these displays allow reliable prediction of the signaller's future behaviour. 2. Skuas were studied on Noss, Fair Isle and Hoy over 3 seasons. Data from 5 colony-year samples were analysed separately. For each interaction, the display used, the signaller's action after displaying, and the receiver's response, were recorded. 3. Attack could be predicted less well than escape. However, variations between samples were so great that neither the absolute nor the relative probabilities of attack or escape could be reliably estimated from the display. 4. The bird that initiated the interaction was more likely to attack or stay, and less likely to escape, than its rival, but the relative probability of attack or escape after different displays was consistent, for birds in the two roles, within a sample. 5. By correlating the responses of the receiver with information about the signaller's future behaviour encoded in its display, it is possible to find whether this information is transferred in the interaction. There was variation between samples in the pattern of correlations, and no consistent evidence of information transfer could be found. 6. The data are discussed in the light of CARYL'S (1979) earlier discussion of models from games theory. The results show that a test of consistency is crucial for any hypothesis about the message carried by a particular display. They indicate that skua displays do not communicate intention in these interactions.

In: Behaviour