Patterns of Aggression in Gulls: Asymmetries and Tactics in Different Social Categories

Abstract

During contests in Western Gulls, Larus occidentalis, we examined patterns of aggression in relation to sex, age class, and territorial status. Data were collected on agonistic contests during parts of three reproductive seasons on Southeast Farallon Island. This colony has dense nesting territories and appears to be near saturation, making breeding space a limiting and valuable resource. Established, territory-holding male gulls were able to defeat nearly all opponents. The exceptions were a few unestablished adult males early in the reproductive period that appeared to be equal, or superior to, established males in fighting ability and persistence. Established females were able to defend their territories against territory-holding neighbors, immature gulls, and adult female intruders, but often were forced to retreat by unestablished male intruders. Both immature and unestablished adult intruders more readily challenged an established female than an established male. The value of breeding territories, the primary contested resource, appeared to increase throughout the reproductive period as established birds became increasingly inclined to escalate encounters after eggs were laid, and even more inclined to escalate after chicks appeared. In contrast, unestablished adults became less common and, when present, less inclined to escalate as the season progressed.