FACTORS INFLUENCING RATE AND SUCCESS OF INTRASPECIFIC KLEPTOPARASITISM AMONG KELP GULLS (LARUS-DOMINICANUS)

Intraspecific kleptoparasitism among Kelp Gulls (Larus dominicanus) wa s studied at four sites in the southwestern Cape Province of South Afr ica. Sites included: (1) undisturbed foraging habitats; (2) a sandy be ach and a rocky shore; and (3) areas where supplementary food was avai lable-a fishing harbor and a refuse dump. Simple food-choice experimen ts were used to test hypotheses generated from field observations. Amo ng-site variation in the rate and success of kleptoparasitism was rela ted to prey attributes, of which prey size and handling time were the most important. In food-choice experiments, gulls selected small prey with short handling times. Prey with long handling times were the most likely to be stolen and the rate of keptoparasitism was higher when p rey were dispersed than when they were clumped. There were marked age- related differences in the rate, although not the success, of kleptopa rasitism among Kelp Gulls. Juvenile (first-year) gulls attempted klept oparasitism significantly more often than expected and adults signific antly less often. Subadults kleptoparasitized in proportion to their a bundance in the population. If an age-related dominance hierarchy exis ts, it mediates kleptoparasitic behavior in Kelp Gull assemblages thro ugh older birds avoiding kleptoparasitic attacks rather than initiatin g them. Simple mathematical models, based on data collected during fie ld observations, were used to investigate the conditions explaining th e rate of intraspecific kleptoparasitism within Kelp Gull populations. Either few individuals can kleptoparasitize relatively frequently, or many individuals can kleptoparasitize infrequently. Apparently, both mechanisms operate within Kelp Gull populations because individuals at tempt kleptoparasitism relatively frequently when they are juveniles a nd inefficient hunters, but infrequently once they are adult and effic ient hunters. The viability of facultative intraspecific kleptoparasit ism as a foraging technique relies on stolen prey being larger on aver age than the prey captured by hunting.