Summary

1. 1. Five pigeons were trained to fly in a boundary-layer wind-tunnel at a velocity of 10 m s⁻¹ for at least 10 min, and a number of respiratory and cardiovascular variables were recorded. For comparison, heart rate, respiratory frequency and E.M.G. from the pectoralis major muscles were also recorded, using radio-telemetry, from free-flying pigeons.

2. 2. For the flights in the wind tunnel there were immediate increases in respiratory frequency and heart rate upon take-off; these variables continued to increase during the flight, eventually becoming on average 411 breaths min⁻¹ (20 × resting) and 670 beats min⁻¹ (6 × resting) respectively. There was a 1:1 relationship between ventilation and wing beat. Oxygen uptake and carbon dioxide production reached their highest values of 12.5 × and 14.4 × resting respectively within 1 min of take-off and then declined to steady levels of 200 ml kg⁻¹ min S.T.P.D. (10 × resting) and 184 ml kg⁻¹ min S.T.P.D. (10.7 × resting) 4 min after take-off. If allowances are made for the weightand drag of the V_OO2 mask and tubes, these stable values are at least 12% higher than would occur in an unloaded bird. Body temperature rose steadily after take-off, reaching a stable value of 43.3°C, which was 2°C above resting, after 6 min of flight. There was a 1.8 × rise in a -v_OO2 content difference and little change in cardiac stroke volume during flight, so that the rise in heart rate was the major factor in transporting the extra O₂ to the active muscles. Respiratory quotient rose from 0.85 at rest to 0.99, 30 s after take off, and then fell to 0.92 after 7 min of flight. Blood lactate rose to 59.8 mg% (6.5 × its resting value).

3. 3. Comparisons with the free-flying birds indicated that the pattern of flight in the wind tunnel was somewhat abnormal, especially at the beginning of a flight, and this may account for the value of V_OO2 being higher at the start of a flight and then declining to a steady value as the flight progressed.

4. 4. Upon landing, heart rate, V^·_O1 V^·_CO2 and body temperature began to fall immediately, and within 2 min, heart rate, V^·_O2 and V^·_CO2 had returned to the ‘tunnel on’ resting values. Respiratory frequency increased upon landing and its decline closely matched the fall in body temperature. R.Q. rose above unity immediately upon landing as CO² was removed in excess of its metabolic production, and then fell below the resting value as CO² was retained, presumably to maintain acid/base balance during the metabolism of lactic acid.