Factors Influencing Rate and Success of Intraspecific Kleptoparasitism among Kelp Gulls (Larus dominicanus)

• Published in: The Auk 1995-10, Vol.112 (4), p.847-859
• Main Authors: STEELE, W. K, HOCKEY, P. A. R
• Format: Article
• Language: English
• Subjects: Age structure; Animal and plant ecology; Animal, plant and microbial ecology; Animals; Autoecology; Aves; Biological and medical sciences; Fish; Food availability; Food intake; Foraging; Fundamental and applied biological sciences. Psychology; Harbors; Kleptoparasitism; Larus dominicanus; Marine; Parasite hosts; Scavenging; Sea birds; Vertebrata
• ISSN: 0004-8038; 1938-4254
• DOI: 10.2307/4089017

Intraspecific kleptoparasitism among Kelp Gulls (Larus dominicanus) was studied at four sites in the southwestern Cape Province of South Africa. Sites included: (1) undisturbed foraging habitats; (2) a sandy beach and a rocky shore; and (3) areas where supplementary food was available-a fishing harbor and a refuse dump. Simple food-choice experiments were used to test hypotheses generated from field observations. Among-site variation in the rate and success of kleptoparasitism was related 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 prey were dispersed than when they were clumped. There were marked age-related differences in the rate, although not the success, of kleptoparasitism among Kelp Gulls. Juvenile (first-year) gulls attempted kleptoparasitism significantly more often than expected and adults significantly less often. Subadults kleptoparasitized in proportion to their abundance in the population. If an age-related dominance hierarchy exists, it mediates kleptoparasitic behavior in Kelp Gull assemblages through older birds avoiding kleptoparasitic attacks rather than initiating them. Simple mathematical models, based on data collected during field observations, were used to investigate the conditions explaining the 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 attempt kleptoparasitism relatively frequently when they are juveniles and inefficient hunters, but infrequently once they are adult and efficient hunters. The viability of facultative intraspecific kleptoparasitism as a foraging technique relies on stolen prey being larger on average than the prey captured by hunting.