DYNAMICS AND EFFECT OF WESTERN GULLS FEEDING IN A COLONY OF GUILLEMOTS AND BRANDTS CORMORANTS

1. In 1981-83, an estimated 2-2% of the male western gulls Larus occidentalis (Audubon) breeding on South-east Farallon Island, California, maintained feeding territories within breeding colonies of guillemots Uria aalge (Pont.) and Brandt's cormorants Phalacrocorax penicillatus (Brandt). These 'specialist' gulls feed by means of scavenging, piracy and predation. 2. This paper investigates specialist's survival and productivity, fidelity to feeding territories, foraging dynamics, energy consumption, and the effect of predation by these gulls on the breeding success of guillemots and cormorants. 3. Specialists were highly adapted, as indicated by their higher reproductive rate and similar or higher survival rate, compared with non-specialists. 4. In years of favourable oceanic food supply, the number of specialists exploiting guillemots and cormorants was consistent because specialists: (i) occupied the same feeding territories each year, (ii) territory boundaries changed little when they died and were replaced; (iii) excluded other gulls from their territories; and (iv) generally fed as opportunists. An exception occurred when oceanic food supply was reduced, and the ratio of specialists to guillemots and cormorants increased, as did predation on guillemot eggs and chicks. 5. Gull predation rate on guillemot eggs and chicks was affected by the guillemot's timing of breeding, nest-site location, nesting density, the specialist's rate of energy consumption on other food sources in the colony, and indirectly by breeding success of the cormorants. The evidence indicate that effect of each factor was ultimately related to food supply. 6. In years of favourable food supply, gull predation rate on guillemot eggs and chicks was moderate, and appeared to be selective on young and/or less-fit birds. In a food-poor, El Nino year, however, predation by gulls was non-selective and of very high rate. Further study is needed to examine the possible interaction between annual variation in food supply, survival probability of successful vs. unsuccessful guillemot breeders, and gull predation rate/selectivity on guillemot eggs and chicks as a possible evolutionary constraint on breeding effort of guillemots.