Aggressive Communication By Lar Us Gla Ucescens Part IV : Experiments On Visual Communication

Abstract

The results of dummy-experiments with Glaucus-winged Gulls on the selection of aggressive display postures for attack, the importance of head and neck level versus the angle of the head with the neck in the discrimination between postures, and the effect of head and neck movement on the response to automated models, were included in this study. It was found that when a pair of models was placed in a territory, the model mounted in a posture with a higher head-neck level, would consistently be attacked first. These results confirmed the importance of head-neck level as a visual cue used in discriminating between the postural components of aggressive displays. It was further demonstrated that the angle of the head with the neck (bill angle) was a relatively unimportant visual cue used in descrimination between postures. Experiments with models capable of making head-neck movements controlled by the experimenter demonstrated that responding gulls reacted to the model from a greater distance, moved away more frequently, and attacked less frequently when the model head-neck was in a low (horizontal) position. These results suggested that the observed greater frequency of attack on static models having a higher head-neck position was due to an inhibition of attack by models with a low head-neck positions. Thus, it was hypothesized that the displays using these low head-neck positions conveyed greater threat (as defined by STOUT, 1975), and would be more effective in preventing attack and/or hastening the escape of the bird toward which they were directed.