Nutrient Reserve Difference between Young and Adult Short· Tailed Shearwaters, Puff/nus Tenu/rostr/s, before and after Trans-equatorial Migration

The Short-tailed Shearwater, Puffinus tenuirostris, is a trans-equatorial migrant, breeding in Tasmania and other parts of southeastern Australia during the Austral summer and migrating to the northern North Pacific for the Boreal summer. Juveniles, in particular, suffer considerable mortality during the migration. Body composition of juveniles and adults in terms of body mass, lipid content, pectoral muscle mass and bone marrow content, before and after migration was investigated. Juveniles varied greatly in body composition preand post-migration and took longer to recover their nutritional status than adults. Adults sustained a better body composition, showing a moderate decrease in body fat upon arrival in the north and recovering their body mass and lipid supply during their stay. The lipid content of beachcast bird carcasses (mean, 4 g) was the minimum necessary for cell membranes and was not metabolisable for energy. Pectoral muscle protein remained high among birds under hyper-nutritional conditions, and decreased gradually at first as lipids decreased, and then rapidly at late malnutritional stages when most lipids had been utilised for energy. Although adults sustained their nutritional status, the amount of lipids in adults leaving the Tasmanian colony was insufficient to accomplish the long-distance migration, so post-breeding adults probably first visited Antarctic waters to accumulate fat reserves before commencing their northward migration.