THE REGULAR ALTERNATION OF SHORT AND LONG FORAGING TRIPS IN THE BLUE PETREL HALOBAENA-CAERULEA - A PREVIOUSLY UNDESCRIBED STRATEGY OF FOOD PROVISIONING IN A PELAGIC SEABIRD

1. The food-provisioning strategy of a pelagic seabird, the blue petrel Halobaena caerulea was assessed on Kerguelen Island by the simultaneous recording of individual trip duration, feed mass and change in adult mass. Dietary analysis provided information on the energetic value of meals and on the spatial distribution of adults at sea. 2. Males and females brought large meals (average 62 g), representing 36% of their unladen mass, and contributed equally to the feeding of the chick. 3. Adults regularly alternated long trips (LT, average duration 7.2 days) over oceanic waters with short trips (ST, 1.8 days) probably restricted to the Kerguelen shelf. They systematically gained mass during LT. but lost this stored energy during the following ST. 4. Quantitative estimation of the energetic yield of trips indicate that although the energy flow per unit of time to the chick is higher after ST than after LT, LT were more advantageous than ST (+1.05 W vs. -0.28 W) which is consistent with a pelagic way of life. Theoretical factors are proposed to explain this result. It is suggested that ST enable the bird to increase the frequency of chick feedings, and are made possible by the use of energy stored during LT. 5. Adult mass may be regulated so that it does not go below a mass threshold, which may prevent adults from paying a survival cost of their short term mass variations.