Summary

1. Oxygen consumption of 2 budgerigars (Melopsittacus undulatus) was measured during level, ascending and descending nights lasting 5-20 min. in a wind-tunnel at speeds between 19 and 48 km./hr. In level flight oxygen consumption was lowest at 35 km./hr. with a mean value of 21.9 ml. (g. hr.)^-1 or 12.8 times the standard value calculated for these birds (weight = 35 g.). At a given speed oxygen consumption was highest for ascending flight and lowest for descending flight.

2. Carbon dioxide production was measured on one bird flying level at 35 km./hr.for 20 min. The ratio of carbon dioxide production to oxygen consumption was 0.780, indicating that the bird was oxidizing primarily fat.

3. The efficiencies of level, ascending and descending flight are discussed. The measurements indicate that for the budgerigar 42 km./hr. is the most economical speed for covering distance, and below 27 km./hr. undulating flight is more economical than flight at a constant altitude.

4. Evaporative water loss in level flight was measured in two birds for 20 min. at 35 km./hr. at temperatures of 18-200 and 29-31° C. At 36-37° C. the birds became overheated and would not fly for as long as 20 min. Evaporative water loss at 18-20° C. was 20.4 mg. (g. hr.)^-1. It increased to 63.9 mg. (g. hr.)^-1 at 36-37° C. After accounting for metabolic water production and faecal water loss, budgerigars flying at 18-20°C. had a net water loss of 11 mg. (g. hr.)^-1. At this temperature 15% of the estimated heat production in flight was lost by evaporation of water, while 47% was lost by evaporation of water at 36-37°C.

5. Lung ventilation, tidal volume and partial pressure of carbon dioxide in expired air were estimated for flying budgerigars from evaporative water-loss data. In level flight at 18-20° C and 35 km./hr. these quantities had values of 398 ml. (g. hr.)^-1, 0.033 ml. (g- breath)^-1 and 37 mm. Hg. respectively.

6. Respiratory rate in level flight was measured in 2 birds at speeds between 19 and 48 km./hr. Respiratory rate depended on speed and was lowest at 35 km./hr. Since wing-beat frequency was constant at 840 beats/min. at all speeds, respiratory rate and wing-beat frequency were not synchronized. Published data and analysis of dimensional relations of birds suggest that in birds the size of a budgerigar or smaller a respiratory rate equal to the wing-beat frequency would be too high for efficient ventilation of the lungs. Birds the size of a pigeon or larger probably have synchronous wing beats and respirations.