AGGRESSIVE COMMUNICATION BY LARUS GLAUCESCENS PART VII. THE ROLE OF THE INTRUDER

Aggressive communication between territory defenders and intruders in a glaucous-winged gull (Lams glaucescens) colony was analyzed using 453 aggressive interactions (strings) recorded on video tape. Special attention was given to the role of the intruder in each interaction. Based on these analyses, the display we named Slouch was described both posturally and functionally. Slouch posture was used almost exclusively by intruders. Each string analyzed was placed in one of two categories, defined by intruder behavior. The category called "Upright" was characterized by intruders using only Upright postures. The second category, called "Slouch", was characterized by intruders that used both Upright and Slouch postures. Analyses were performed separately but identically on strings from each category. Conclusions were based on comparisons of results from the two categories. Interactions in both categories were initiated by an intruder entering a territory. Territory owners responded to intruder entry and effected intruder exit in a virtually identical fashion in both categories. This indicated differences between Slouch and Upright categories were derived from differences occurring during the interim between intruder entry and prior to intruder exit. The interim period in Slouch category interactions was temporally more protracted than in Upright category interactions. However, total behavior frequency was approximately equal in both categories. These data indicated pauses were occurring in the intermediate segments of the lengthier Slouch strings. Pauses during the interim corresponded with episodes of intruder Slouch and subsequent defender responses. Intruders used Slouch independent of preceding defender behaviors. Whereas the behaviors of territory defenders tended to be predictably associated with specific behaviors of Upright intruders, responses to intruders using Slouch were characterized by a variety of non-associated behaviors. This lack of predictability was judged to result from tenacious intruder adherence to Slouch regardless of defender response, and resulted in creating the long pauses in Slouch strings. This is a likely indicator of a more threatening intruder. Previous experimental work has shown that Upright intruders approaching a defender are responded to as being more threatening than those remaining stationary or moving away. An analysis of interactions in which both Upright intruders and those using Slouch approached the defender revealed almost identical defender responses. Furthermore, intruders using Slouch approached territory defenders more frequently than did Upright intruders, and responded classically less frequently to defender Mew-Choke display than did Upright intruders. Both characteristics are indicative of a higher level of tenacity (or threat) among intruders using Slouch. Differences observed in the two categories of strings were interpreted as being intruder specific, since these differences were based on the use or lack of use of Slouch display by intruders. Furthermore, the fact that territory defenders responded to initial entry of intruders in both categories in the same fashion was used to rule out differences as being defender specific. The observed differences appeared to result from a single factor-the seeming inability of territory owners to rapidly displace intruders using Slouch display. It was concluded the chief differences between Upright and Slouch intruders was the degree of threat conveyed by each display to the territory defender. Intruders using Slouch were more site tenacious, they conveyed a greater level of threat than did Upright intruders, and consequently were able to remain longer in a territory.