D5 Flight Respirometry

Topic

Open-flow respirometry in an ornithological wind tunnel is a method that allows for the direct measurement of a bird's metabolic power during flight, rather than inferring it indirectly. The bird flies under controlled conditions within the wind tunnel while the equipment continuously measures its oxygen consumption and carbon dioxide production at every moment of the flight.

Based on the oxygen consumption and carbon dioxide production measured during flight, open-flow respirometry yields two distinct results: the total metabolic power the bird expends to sustain that flight, and the energy substrate the bird is utilizing at that moment—fat, carbohydrate, or protein. This substrate is identified by calculating the respiratory quotient from the ratio of oxygen consumed to carbon dioxide produced.

By comparing the measured metabolic power curve (plotted against flight speed) with the curve predicted by the bird's theoretical model, open-flow respirometry allows for the calibration of species-specific parameters required by the model—such as the mechanical efficiency with which muscle converts metabolic energy into useful flight power, or the drag coefficient of the bird's body. These values ​​vary between species and cannot be determined beforehand without direct measurement of this kind.

This same comparison makes it possible to detect anomalies in the bird being measured: when the measured metabolic power exceeds the theoretical model's prediction for a given flight speed, the difference indicates that the bird is expending more metabolic energy than expected to generate the same amount of useful flight power. This finding is consistent with either pectoral muscle damage or inefficiency in the tendon system, where the tendons fail to return elastic energy to the muscle as a healthy tendon would. Open-flow respirometry also detects a second anomaly via the respiratory quotient: while the bird is burning only fat, the respiratory quotient remains close to the value characteristic of that lipid phase; however, when the quotient begins to shift toward values ​​between 0.8 and 0.9, this indicates that the bird has started burning protein as well—marking the onset of protein catabolism and the transition to the mixed phase of energy reserve utilization.

In addition to calibrating the simulator and detecting these anomalies, wind-tunnel open-flow respirometry serves another purpose: it is the method used to calibrate cardiac telemetry for field use. Before deploying a heart-rate logger on a free-living bird, it is necessary to establish the relationship between the bird's heart rate and its metabolic power—doing so within the wind tunnel while simultaneously measuring actual metabolic power via open-flow respirometry. Only after this relationship has been established in the wind tunnel can heart rate measured in the field be used as a non-invasive, indirect indicator of the metabolic power the bird expends during actual flight.

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