Nest-Site Selection in South Polar Skuas: Balancing Nest Safety and Access to Resources

Abstract

-Although South Polar Skuas (Catharacta maccormicki) are important predators of Adelie Penguin (Pygoscelis adeliae) eggs and chicks, skuas experience both reproductive costs and benefits when nesting near Adelie Penguins. We present a conceptual model to show that skua nest placement is based on two mechanisms: nest safety and access to penguins. Skua nests close to of breeding penguins are likely to suffer 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 reproductive tradeoffs, skuas should exhibit an optimal nesting distance relative to penguin subcolonies. Skua pairs located at the optimal distance minimize egg loss while maximizing access to penguin eggs and chicks. During four breeding seasons we monitored skua nest placement relative to 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 were unsafe and lost eggs more frequently than those farther away. Once hatched, chicks were more likely to fledge in nests closer to penguin subcolonies. Furthermore, skua pairs may assess the quality of their nest site and adjust nesting locations between years. During 1991, skuas whose eggs had failed to hatch in 1990 nested significantly farther from their 1990 nest location and chose safer nest sites than did skuas that had nested successfully in 1990. Received 8 July 1996, accepted 10 April 1997. SOUTH POLAR SKUAS (Catharacta maccormicki) often breed near Adelie Penguin (Pygoscelis adeliae) colonies around the coast of Antarctica (Miiller-Schwarze and Miiller-Schwarze 1973, Trillmich 1978). Although it is well known that South Polar Skuas do not require penguin prey for successful reproduction (Young 1963a, b, 1994; Pietz 1987), predation on penguin eggs and chicks provides a substantial food resource for adult skuas and their offspring (Young 1963a, b, 1994; Spellerberg 1971; Mund and Miller 1995), and this food resource probably attracts skuas to penguin colonies. South Polar Skuas may nest around the perimeter of Adelie Penguin colonies (e.g. Cape Hallet; Trillmich 1978) or between groups or subcolonies within a penguin colony. The reproductive success of skuas is both hindered and facilitated by their association with penguins. Skuas that nest near penguins often are attacked by them. These attacks often result in the skua being driven from its nest and the penguin subsequently stepping on the skua eggs and breaking them (Young 1963a, 1994; this study). South Polar Skuas also are well-known predators on each other's eggs and chicks (Young 1963a, 'E-mail: jhagelin@unm.edu 1994; Spellerberg 1971). Despite these negative effects, skuas continue to nest near penguin and often maintain foraging areas that contain breeding Adelie Penguins (Young 1963a, b, 1994; Trillmich 1978). Because South Polar Skuas experience reproductive costs and benefits based on the distance of their nests to penguin subcolonies, we propose the following conceptual model of skua nest placement (see Fig. 1). Skuas should nest far enough from penguin to gain some safety from nest trampling by penguins and predation by conspecifics, but close enough to have access to penguins for food. Skua reproductive success (i.e. number of young fledged) depends on: (1) the safety of a nest in terms of vulnerability to egg loss, and (2) access to penguin resources that the nesting location provides to skuas. Skua nests close to a penguin subcolony (or near paths regularly used by penguins) are not as safe as those farther away. Therefore, the safety function has a positive slope relative to the distance from penguins (Fig. 1). Because the intensity of penguin traffic is greatest immediately adjacent to subcolonies, the safety function for skua nests should not be linear (Fig. 1). When nest safety is low, egg loss may