Foraging behaviour and reproductive success in Magellanic penguins (Spheniscus magellanicus):: a comparative study of two colonies in southern Chile

During the breeding season 1996/97 we compared the foraging and diving behaviour of adult Magellanic penguins (Spheniscus magellanicus), growth rates of their chicks and their breeding success at two colonies in the south of Chile. One of the colonies is located on Magdalena Island in the Strait of Magellan, where a commercial fishery existed several years ago; the other, on the shores of the yet unexploited Otway Sound. Thirty adult Magellanic penguins were equipped with time-depth recorders (TDR) to investigate their behaviour at sea. In each colony 15 adults returning from the sea were stomach flushed to analyse dietary composition. Chicks of TDR-nests and of 12 additional control nests were weighed regularly. Foraging effort was significantly higher at Magdalena than at Otway. The Magdalena-birds usually remained at sea overnight and foraged with a mean duration of 18 h, whereas the penguins of Otway Sound foraged during 1-d trips with a mean duration of only 9 h. Compared to Magdalena, penguins at Otway dived shallower (mean depth 14.9 vs 16.5 m), shorter (mean duration 57.8 vs 64.3 s) and showed more searching and feeding as opposed to travelling activity ton average 69 vs 55%) during the foraging trips. Compared to other breeding locations both colonies were characterised by high chick growth rates, high fledging body masses (>3 kg) and early fledging date (after 70 to 80 d), and a very high reproductive success of >1.75 chicks per breeding pair. Comparison of the diet (almost exclusively sprats) with former investigations suggests for both areas an unchanged food structure over the last decade. The results in both colonies indicate ample food availability in the season 1996/97. However, compared to the much smaller Otway colony, penguins on Magdalena have to cope with more competition for food. Therefore, future prey limitation, through resumed fishery operations or effects of El Nino, might affect the penguin population on the island more negatively than in Otway Sound.