THE ENERGY-EXPENDITURE OF FREE-RANGING BLACK-BROWED ALBATROSSES

As heart rate Vk) can be used to determine the energy expenditure of b lack-browed albatrosses (Diomedea melanophrys) (Bevan et al. 1994), da ta loggers - recording f(H) and abdominal temperature (T-ab) - were im planted into free-ranging black-browed albatrosses breeding at South G eorgia. Five birds also had salt water switches (sws) attached to one leg to record when the birds were on the water, and two others had sat ellite transmitters attached to their back to determine the birds' pos ition at sea. The birds were released into their natural environment a nd recaptured, on average, 23 days later when the data loggers were re moved. The f(H) data were then converted into estimates of energy expe nditure (EE) using a previously derived equation. The mean EE during i ncubation and brooding were 2.22 and 2.42 W kg(-1), respectively. When the birds were foraging at sea, EE increased to between 4.63 and 5.80 W kg(-1), depending on the phase of the reproductive cycle. As the bi rds spent approximately the same length of time at the nest and at sea during incubation and brooding, the overall mean EE during these phas es were 3.63 and 3.54 W kg(-1) respectively. These rates are significa ntly lower than that during the chick-rearing phase when a high level of foraging EE is maintained almost continuously. By combining informa tion from the sws with the f(H) data, it was possible to determine the EE of the birds when on the water (5.77 W kg(-1)) and when flying (6. 21 W kg(-1)). These values are approximately twice the estimated basal metabolic rate (BMR) for the species. The energy costs of flight are half previous values, estimated using the doubly labelled water techni que, because of the previous assumption that birds on the water have a n EE equivalent to BMR. When the birds were on the nest, T-ab was 39.3 +/- 0.4 degrees C and this changed very little with time. However, wh en they were at sea, T-ab showed large variations, depending on the be haviour of the bird. Information from the sws indicated that all large drops (> 0.5 degrees C) in T-ab occurred when the birds were on water . The mean minimum value reached was 32.5 +/- 2.0 degrees C. It is lik ely that ingestion of prey or water are the major causes of this decre ase. This is the first study to have used f(H) extensively to determin e the EE of a free-ranging marine bird. The advantages of using this t echnique are that data can be obtained over long durations with high r esolution, permitting the EE Of different activities to be estimated.