1. The fish crow and the laughing gull, although similar in body mass, differ in wing morphology and wingbeat frequency. We studied the power input during sustained flight in the fish crow for comparison with data reported for the laughing gull.

2. Two fish crows (mean mass 275 g) were trained to fly in a wind-tunnel for 15-20 min at air speeds of 7-11 m s-1 in descending flight at angles of 2°, 4° and 6° relative to horizontal. Oxygen consumption was determined by analysing air sampled through a mask and attached trailing air tube. Power input was defined as the rate of metabolic energy expenditure calculated from rate of oxygen consumption. The results were corrected for the drag of the mask and tube, and power input for horizontal flight was calculated.

3. Power input varied little with air speed, but decreased with increasing angle of descent. Minimum power input for horizontal flight (0.083 W g-1) was 6.4 times greater than mean resting power input. During horizontal flight, power input for the fish crow exceeded by about one-third that for the laughing gull.

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