RELATIONSHIP BETWEEN FORAGING EFFORT AND ENERGY REQUIREMENT THROUGHOUT THE BREEDING-SEASON IN THE WANDERING ALBATROSS

1. Pelagic seabirds feeding far from their nest are likely to have to adjust their foraging effort to environmental constraints to meet their energy requirements. Foraging effort of wandering albatrosses, measured as distance travelled daily and flight velocity, was studied throughout the I-year breeding season using satellite telemetry and compared to a model of the foraging effort required to cover breeding costs at the successive stages of the breeding cycle. 2. Foraging effort was greatest during the long pelagic trips of the hedging period, when birds covered daily the longest distances and had the highest flight velocities, and lowest during the late incubation period. Birds are able to increase their foraging effort by flying at night during moonlight. Foraging effort increases with day-length and wind speed. Females fly farther than males during incubation and brooding. 3. The daily amount of food necessary to fulfil the power requirement at sea is highest during the brooding period and the lowest after the chick has been left alone on the nest. The model suggests that foraging effort should nonetheless be highest after the chick has been left alone because wandering albatrosses brood their chick when food is most available, but rear fledglings during the less favourable period. 4. The predictions of the model appear to fit well with our results and suggest that the timing of the most constraining period in terms of energy requirement, the brooding period, occurs during the period of supposed highest prey abundance. 5. Birds are able to cope with the highest demand for foraging effort during the less favourable period in terms of food availability and duration of the day-length by increasing their foraging effort through night foraging. 6. The possible causes for the contrasting foraging efforts observed in males and females are discussed and it is suggested that they result from the segregation of feeding zones between the two sexes.