Hockey P.R., Ryan P.G. & Bosman A.L. (1989) Age-related intraspecific kleptoparasitism and foraging success of Kelp Gulls Larus dominicanus. ARDEA 77 (2): 205-210

The relationship between hunting efficiency and intensity of intraspecific kleptoparasitism in Kelp Gulls Larus dominicanus was studied at Las Cruces beach, Chile, where the gulls preyed almost exclusively on the sand mussel Mesodesma donacium. The efficiency of foraging behaviour increased with age. As hunting and handling success increased, the incidence of kleptoparasitism diminished, although for all age classes the rewards of kleptoparasitism were greater than those of hunting. Immature birds lost many prey items to parasites because of their less efficient prey handling techniques, but largely offset these losses by gains from their own kleptoparasitic behaviour. A mixed foraging strategy, balancing kleptoparasitism and hunting, increased intake rate of immature birds during active foraging by 21%, compared with only 9% for adults. Food was stolen from birds of all age classes with equal success. Differential levels of kleptoparasitism within demographic subsections of the gull population satisfies a primary criterion for the evolutionary stability of this behaviour.