Causation and Function of Different Vocal Reactions of Little Gulls Larus Minutus To Intruders Near the Nest

Abstract

[This study deals with (1) which vocalizations are made by little gulls (Larus mireutus) during agonistic interactions and (2) how these are related with overt attack and escape behaviour. Under natural circumstances, when little gulls react to intruders approaching the nests, five different call types can be heard. Four of these, Tuk, Kee, Eeyit and Whet, often occurred during experiments using stuffed intruder models (little gull and black-headed gull). Reactions of incubating little gulls to a stuffed conspecific placed near the nests, showed that the occurrence of different call types is related to different phases of the flight pattern, such as circling above the intruder, approaching, attacking and moving away. Reactions to a stuffed little gull and black-headed gull which were moved towards and away from the nests revealed that the call types Whet, Eeyit, Kee and Tuk, were, in decreasing order, associated with actual attack. Consistent individual differences existed in the reactions to the experimental intruders. The movements and the vocalizations made by the gulls when reacting to the experimental intruders are explained as a consequence of a conflict between motivational systems for attack and escape. It is argued that different call types are related with different strengths of activation of these systems. Differences between individuals in the reactions to the intruders can largely be explained by individual-typical differences in motivation. A comparison of the physical structure of each type of vocalization in relation to its underlying motivational state, shows a positive relationship between call duration and attack motivation. Tuk is believed to function as a warning for neighbouring birds, chicks and the mate. Eeyit seems to convey 'I shall attack, if provoked' and is intended for the intruder. Whet probably acts as a vocal part of the actual attack and may help in impressing and distracting the intruder. The functional significance of Kee is as yet unknown., This study deals with (1) which vocalizations are made by little gulls (Larus mireutus) during agonistic interactions and (2) how these are related with overt attack and escape behaviour. Under natural circumstances, when little gulls react to intruders approaching the nests, five different call types can be heard. Four of these, Tuk, Kee, Eeyit and Whet, often occurred during experiments using stuffed intruder models (little gull and black-headed gull). Reactions of incubating little gulls to a stuffed conspecific placed near the nests, showed that the occurrence of different call types is related to different phases of the flight pattern, such as circling above the intruder, approaching, attacking and moving away. Reactions to a stuffed little gull and black-headed gull which were moved towards and away from the nests revealed that the call types Whet, Eeyit, Kee and Tuk, were, in decreasing order, associated with actual attack. Consistent individual differences existed in the reactions to the experimental intruders. The movements and the vocalizations made by the gulls when reacting to the experimental intruders are explained as a consequence of a conflict between motivational systems for attack and escape. It is argued that different call types are related with different strengths of activation of these systems. Differences between individuals in the reactions to the intruders can largely be explained by individual-typical differences in motivation. A comparison of the physical structure of each type of vocalization in relation to its underlying motivational state, shows a positive relationship between call duration and attack motivation. Tuk is believed to function as a warning for neighbouring birds, chicks and the mate. Eeyit seems to convey 'I shall attack, if provoked' and is intended for the intruder. Whet probably acts as a vocal part of the actual attack and may help in impressing and distracting the intruder. The functional significance of Kee is as yet unknown.]