SUMMARY It is technically demanding to measure the energetic cost of animal flight. Each of the previously available techniques has some disadvantage as well advantages. We compared measurements of the energetic cost of flight in a wind tunnel by four European starlings Sturnus vulgaris made using three independent techniques: heat transfer modelling, doubly labelled water (DLW)and mask respirometry. We based our heat transfer model on thermal images of the surface temperature of the birds and air flow past the body and wings calculated from wing beat kinematics. Metabolic power was not sensitive to uncertainty in the value of efficiency when estimated from heat transfer modelling. A change in the assumed value of whole animal efficiency from 0.19 to 0.07 (the range of estimates in previous studies) only altered metabolic power predicted from heat transfer modelling by 13%. The same change in the assumed value of efficiency would cause a 2.7-fold change in metabolic power if it were predicted from mechanical power. Metabolic power did not differ significantly between measurements made using the three techniques when we assumed an efficiency in the range 0.11–0.19, although the DLW results appeared to form a U-shaped power-speed curve while the heat transfer model and respirometry results increased linearly with speed. This is the first time that techniques for determining metabolic power have been compared using data from the same birds flying under the same conditions. Our data provide reassurance that all the techniques produce similar results and suggest that heat transfer modelling may be a useful method for estimating metabolic rate.

SUMMARY We tested the hypotheses that the relationship between heart rate( f H) and the rate of oxygen consumption( V̇ O 2 ) differs between walking and flying in geese and that f H and V̇ O 2 have a U-shaped relationship with flight speed. We trained barnacle geese Branta leucopsis (mean mass 2.1 kg) and bar-headed geese Anser indicus (mean mass 2.6 kg) to walk inside a respirometer on a treadmill and to fly in a wind tunnel with a respirometry mask at a range of speeds. We measured f H and V̇ O 2 simultaneously during walking on the treadmill in five individuals of each species and in one bar-headed goose and four barnacle geese during flight in the wind tunnel. The relationships between f H and V̇ O 2 were significantly different between flying and walking. V̇ O 2 was higher,and the increment in V̇ O 2 for a given increase in f H was greater, for flying than for walking geese. The relationship between f H and V̇ O 2 of free-living barnacle geese during their natural migratory flights must differ from that measured in the wind tunnel, since the f H of wild migratory birds corresponds to values of V̇ O 2 that are unrealistically low when using the calibration relationship for our captive birds. Neither f H nor V̇ O 2 varied with flight velocity across the range of speeds over which the geese would fly sustainably.

SUMMARY We trained two starlings ( Sturnus vulgaris ) to fly in a wind tunnel whilst wearing respirometry masks. We measured the metabolic power ( P met ) from the rates of oxygen consumption and carbon dioxide production and calculated the mechanical power ( P mech ) from two aerodynamic models using wingbeat kinematics measured by high-speed cinematography. P met increased from 10.4 to 14.9 W as flight speed was increased from 6.3 to 14.4 m s –1 and was compatible with the U -shaped power/speed curve predicted by the aerodynamic models. Flight muscle efficiency varied between 0.13 and 0.23 depending upon the bird, the flight speed and the aerodynamic model used to calculate P mech . P met during flight is often estimated by extrapolation from the mechanical power predicted by aerodynamic models by dividing P mech by a flight muscle efficiency of 0.23 and adding the costs of basal metabolism, circulation and respiration. This method would underestimate measured P met by 15–25 % in our birds. The mean discrepancy between measured and predicted P met could be reduced to 0.1±1.5 % if flight muscle efficiency was altered to a value of 0.18. A flight muscle efficiency of 0.18 rather than 0.23 should be used to calculate the flight costs of birds in the size range of starlings (approximately 0.1 kg) if P met is calculated from P mech derived from aerodynamic models.

This study examined the effects of oxygen tensions ranging from 0 to 90 kPa on the metabolic rate (rate of carbon dioxide production), movement and survivorship of the free-living soil nematode Caenorhabditis elegans. C. elegans requires oxygen to develop and survive. However, it can maintain a normal metabolic rate at oxygen levels of 3.6 kPa and has near-normal metabolic rates at oxygen levels as low as 2 kPa. The ability to withstand low ambient oxygen levels appears to be a consequence of the small body size of C. elegans, which allows diffusion to supply oxygen readily to the cells without requiring any specialized respiratory or metabolic adaptations. Thus, the small size of this organism pre-adapts C. elegans to living in soil environments that commonly become hypoxic. Movement in C. elegans appears to have a relatively minor metabolic cost. Several developmental stages of C. elegans were able to withstand up to 24 h of anoxia without major mortality. Longer periods of anoxia significantly increased mortality, particularly for eggs. Remarkably, long-term exposure to 100 % oxygen had no effect on the metabolic rate of C. elegans, and populations were able to survive for a least 50 generations in 100 % (90 kPa) oxygen. Such hyperoxic conditions are fatal to most organisms within a short period.

Infrared thermography was used to measure heat transfer by radiation and the surface temperature of starlings (Sturnus vulgaris) (N=4) flying in a wind tunnel at 6–14 m s-1 and at 15–25 degrees C. Heat transfer by forced convection was calculated from bird surface temperature and biophysical modelling of convective heat transfer coefficients. The legs, head and ventral brachial areas (under the wings) were the hottest parts of the bird (mean values 6.8, 6.0 and 5.3 degrees C, respectively, above air temperature). Thermal gradients between the bird surface and the air decreased at higher air temperatures or during slow flight. The legs were trailed in the air stream during slow flight and when air temperature was high; this could increase heat transfer from the legs from 1 to 12 % of heat transfer by convection, radiation and evaporation (overall heat loss). Overall heat loss at a flight speed of 10.2 m s-1 averaged 11. 3 W, of which radiation accounted for 8 % and convection for 81 %. Convection from the ventral brachial areas was the most important route of heat transfer (19 % of overall heat loss). Of the overall heat loss, 55 % occurred by convection and radiation from the wings, although the primaries and secondaries were the coolest parts of the bird (2.2-2.5 degrees C above air temperature). Calculated heat transfer from flying starlings was most sensitive to accurate measurement of air temperature and convective heat transfer coefficients.