Foraging efficiency and adjustment of energy expenditure in a pelagic seabird provisioning its chick

1. Foraging determines the intake of resources that are expended on fitness related activities, and allocation links foraging and life-history traits, but the extent to which individuals are able to adjust energy expenditure according to allocation decisions is still poorly studied. 2. To examine this, we studied for the first time the efficiency of foraging trips (defined as the ratio of energy gained over energy spent while foraging) in a pelagic seabird provisioning its chick by measuring simultaneously the energy flow to the chick, changes in adult body mass and foraging cost using doubly labelled water. The study was performed over two seasons. 3. Blue petrels (Halobaena caerulea ) alternate between short foraging trips (average 1.9 days, range 1-4 days) where they maximize food delivery to the chick but loose mass, and long foraging trips (7.3 days, 5-10 days) where they increase in mass. They expend 1.5-2.2 times more energy per unit of time during short trips (2561 kJ day(-1) kg(-1) , i.e. 3.2 times the resting metabolic rate) compared to long trips (1142-1676 kJ day(-1) kg(-1) , i.e. 1.4-2.1 the resting metabolic rate). 4. The currency during long trips is therefore to maximize efficiency, whereas during short trips the currency is to maximize rate of energy delivery to the chick. Maximum efficiency peaks for 6-day trips, i.e. when adult mass gain is highest, foraging costs are lowest and efficiency was extremely low during short trips. A duration of 6 days is therefore the optimal duration of long foraging trips but trips of 7 days were more common. 5. Our study shows that self-feeding during long trips is crucial for the success of short trips because most of the energy used during short trips is probably derived from the energy stored during long trips. Self-feeding during short trips is probably negligible. 6. The study indicates that surprisingly long distance foraging may not always result in higher foraging costs compared to shorter movements. It highlights the predominant role of body store usage as a mechanism for this animal to adjust energy expenditure and energetic efficiency during foraging for allocation purposes.