Basal Metabolic Rate in Tropical Seabirds

Abstract

There is evidence that the basal metabolic rates (BMR) of tropical birds are lower than those of birds in higher latitudes (Weathers 1979, Hails 1983). Some species of tropical seabirds appear to have relatively low BMRs, while others do not (MacMillen et al. 1977; Ellis et al. 1982a, b; Ellis 1984). The purpose of our study was to obtain additional data on tropical seabirds. The impetus came from the use of BMRs of tropical seabirds in the estimation of the energy requirements of tropical seabird populations, in which a small error in the BMR of an individual is magnified considerably when multiplied by the number of birds comprising the population (Pettit et al. 1984). A small field laboratory was established on Tern Island, French Frigate Shoals (23?52'N, 165?18'W) in the Northwestern Hawaiian Islands. The 8 species studied (see Table 1) nest on Tern Island and are present throughout much of the year. Experiments were conducted during August and September 1981 and January, February, and July 1982. The BMR was determined from the oxygen consumption of resting birds in a darkened metabolic chamber, during the day, at the air temperature prevailing on Tern Island. Birds were captured by hand, encouraged to regurgitate their stomach contents, and held overnight (15-20 h) to achieve a fasting condition. Oxygen consumption (V.2) was measured by two techniques. In the open-flow technique, ambient air was drawn through a metabolic chamber (volume 38 1 or 11 1 depending on the size of the bird) with a diaphragm pump. Constant flow rates ranged from 0.9 to 3.0 1/min, as determined with a calibrated Brooks rotometer flow meter, the level of flow depending on the size of the chamber. After the bird was placed in the chamber, the first gas sample was obtained following a 60-min equilibration period. Gas samples, collected in glass syringes lubricated with mineral oil, were analyzed in duplicate by a Scholander micro-gas analyzer. All gas values were converted to STPD. For petrels, an 8-1 closed system was employed. Oxygen consumption was measured manometrically after a 45-min equilibration period. Carbon dioxide absorbant (sodasorb) and water-vapor absorbant (silica gel) were placed in the chamber, and a measured volume of oxygen was introduced to maintain the pressure within the chamber at atmospheric pressure. A compensating thermobarometer chamber was not used.