PREDATION OF HEERMANNS GULL (LARUS-HEERMANNI) CHICKS BY YELLOW-FOOTED GULLS (LARUS-LIVENS) IN DENSE AND SCATTERED NESTING SITES

Rasa (Flat) Island is the nesting site for over 90% of the world population of Heermann's Gulls. Breeding is very synchronous and during the first month after hatching, aerial predation by Yellow-footed Gulls on Heermann's Gull chicks is conspicuous. This predation was analyzed and compared at a valley (71 nests/100 m2) and a rocky hill (9.5 nests/100 m2) area. In the valley, the total number of predation events was higher, but the proportion of nests preyed upon was significantly lower. Predatory gulls spend significantly more time searching for prey and made significantly more predation attempts in the valley compared to the rocky hill. The number of successful predation events per unit time a predator was present in the observation area and the success rate of predators did not differ between areas. This behavior fits the predictions of the ''ideal free distribution'' model. During the first month after hatching, a positive correlation was found between prey and predator numbers. Breeding synchronously and densely could thus give the Heermann's Gull an effective defense against this predator on Rasa Island.