Aggressive Communication By Larus Glaucescens Iii. Description of the Displays Related To Territorial Protection

This paper is a description of the aggressive activities of the Glaucous-winged Gull, Larus glaucescens. It provides a descriptive basis for the interpretation of experiments designed to elucidate the aggressive communication of this species (STOUT & BRASS, 1969; STOUT, WILCOX & CREITZ, 1969; and in progress). The descriptions of the aggressive behavior of L. glaucescens were based on the analysis of 12,000 feet of motion picture film, and on the physical analysis of a large number of sound spectrograms (sonograms) of each of the aggressive calls. The behavior observed was classified with respect to its association with attack, escape, or other aggressive interactions, and also with respect to its production by territorial defender or intruder. The Aggressive Upright, Moving Aggressive Upright, Intimidated Upright, Trumpeting, Mew, Grass-pulling, Jabbing, and Choking displays were described. The Forward display, as described for other larids, was not seen. The physical analysis of the aggressive calls of the Glaucous-winged Gull demonstrated great similarity between the Trumpeting and Yelp Calls. The Courtship, Parent-young, and Aggressive Mews were demonstrated to have consistent physical differences. It was suggested that they could each have a different function. The Choking and Alarm Calls are similar in that they have a more complex harmonic structure than the other calls. However, a differential function has already been demonstrated for these two calls (STOUT, WILCOX & CREITZ, 1969). It was hypothesized that each of these aggressive displays communicates distinct levels of threat. The combination of head level (posture), call, orientation, and movement were considered as possible factors resulting in the distinction between displays. It was suggested that the Upright Threat, Trumpeting, Mew, and Choking displays communicate increasing levels of threat in that order. This model was evaluated in relationship to experiments performed on aggressive communication by L. glaucescens.