Abstract
Black-tailed Gulls Larus crassirostris steal fish from Rhinoceros Auklets Cerorhinca monocerata when the latter arrive at their colony to feed their chicks after sunset. The success of this kleptoparasitism depends on the level of ambient light, which decreases rapidly after sunset, and the vegetation of the nesting grounds. The distribution of gulls across habitats may be affected by variation in their success at kleptoparasitism. To examine the factors affecting their success, and its effect on the distribution of the gulls, we recorded the number of auklets arriving with fish, the number of waiting gulls, kleptoparasitic event, and aggressive interactions between gulls in areas with tall grass, low grass and no vegetation during high (>1 lx), middle (0.1–1 lx), and low (<0.1 lx) light levels on Teuri Island. The proportion of the auklets attacked by gulls was high in the habitats with low grass and no vegetation during periods of high light level. Among the auklets that were attacked by gulls, more birds lost fish when they were attacked by a larger number of gulls under high light level. As a larger number of gulls waited in the area with no vegetation, the success of kleptoparasitism per gull was low there, and high during high light level in habitats with tall and low grass. Unexpectedly, the number of waiting gulls was not related to their success. Aggressive interactions occurred more frequently in the habitats with high success. Observations of individually marked gulls suggested that males employed in kleptoparasitism more frequently did females. Only three out of nine males repeatedly visited habitats giving high success. Rapid changes in the success of kleptoparasitism and individual variation in aggressiveness may affect the distribution of the gulls stealing from auklets.
