Wk. Steele et Par. Hockey, FACTORS INFLUENCING RATE AND SUCCESS OF INTRASPECIFIC KLEPTOPARASITISM AMONG KELP GULLS (LARUS-DOMINICANUS), The Auk, 112(4), 1995, pp. 847-859
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.