UC Santa Cruz

1. Theoretical models predict that animals will vary their effort to maximize different currencies such as time and energy when the constraints of reproduction change during breeding, but this has been poorly studied in free-ranging animals. 2. Foraging effort (energy per unit time) was examined by comparing mass changes, foraging costs and activity-specific behaviours of Wandering Albatrosses (Diomedea exulans Linnaeus) during the incubation and chick-brooding stages. In 1998, 38 albatrosses (20 during incubation and 18 during brooding) were injected with doubly labelled water and equipped with satellite transmitters and activity data loggers. 3. During incubation, albatrosses travelled 3.7 times farther and were at sea 3.2 times longer, yet foraging costs were significantly lower than trips made during brooding (incubation 4.52 +/- 0.50 SD W kg(-1) vs brooding 4.98 +/- 0.55 SD W kg(-1)). 4. The rate of daily mass gain decreased significantly with time at sea during incubation whereas the rate of daily mass gain increased significantly with time at sea during brooding. 5. Foraging effort was higher during brooding, suggesting that birds were minimizing time at sea to maximize the rate of food delivery to chicks. In contrast, foraging effort was lower during incubation, suggesting that birds were maximizing time at sea and minimizing the energy costs of foraging. 6. Foraging costs were also different between sexes. However, this was related to body size differences and not to differences in foraging effort as suggested in previous studies.