Flight Energetics of Free-Ranging Red-Footed Boobies (Sula sula)

Abstract

Flight energetics of free-ranging red-footed boobies (Sula sula) on Johnston Atoll in the central Pacific Ocean were quantified using doubly labeled water to measure field metabolic rate (FMR), an open-flow system to measure O₂ consumption rate, and activity recorders to monitor foraging behavior at sea. Brooding birds spent an average of 496% ± 6% of the time at the nest, 6% ± 2% of the time sitting on the water, and 41% ± 7% of the time in flight. Mean FMR was 14.2 ± 1.2 W (n = 9; mean mass = 1,070 ± 40.3 g). Mean resting metabolic rate was 9. 7 ± 0.7 W (n = 8: mean mass = 1,039 ± 25.8 g). The power required for flight averaged 19.0 ± 3.4 W (n = 6; mean mass = 1, 014 ± 42 g). This was less than one-third of the predicted value (62.1 W) obtained from equations based on aerodynamic theory. Aerodynamic theory predicts that birds in this study would require a mean of 0.5 ± 0.01 m s−1 of lift in order to be able to soar. Wind velocity during the period of FMR measurement averaged more than 10 times this, 5.7 ± 0.3 m s−1. Thus, this striking difference between measured andpredicted cost of flight is likely due to the ability of birds to soar, and, in fact, observations of red-footed boobies at sea confirm that their flight includes a complex set of behaviors that probably serve to take advantage of wind energy and substantially lower their energetic cost of flight. More data are needed on flight behavior patterns of birds at sea.