NEST-SITE SELECTION IN SOUTH POLAR SKUAS - BALANCING NEST SAFETY AND ACCESS TO RESOURCES

Although South Polar Skuas (Catharacta maccormicki) are important pred ators of Adi lie Penguin (Pygoscelis adeliae) eggs and chicks, skuas e xperience both reproductive costs and benefits when nesting near Adeli e Penguins. We present a conceptual model to show that skua nest place ment is based on two mechanisms: nest safety and access to penguins. S kua nests close to subcolonies of breeding penguins are likely to suff er greater egg loss due to egg trampling by penguins and predation by other skuas. However, skuas nesting near penguins potentially benefit from direct access to penguin eggs and chicks. Given these reproductiv e tradeoffs, skuas should exhibit an optimal nesting distance relative to penguin subcolonies. Skua pairs located at the optimal distance mi nimize egg loss while maximizing access to penguin eggs and chicks. Du ring four breeding seasons we monitored skua nest placement relative t o breeding penguins and recorded the fate of all skua eggs and chicks. The results supported the safety and access mechanisms of our model. Skua nests close to penguin subcolonies were unsafe and lost eggs more frequently than those farther away. Once hatched, chicks were more li kely to fledge in nests closer to penguin subcolonies. Furthermore, sk ua pairs may assess the quality of their nest site and adjust nesting locations between years. During 1991, skuas whose eggs had failed to h atch in 1990 nested significantly farther from their 1990 nest locatio n and chose safer nest sites than did skuas that had nested successful ly in 1990.