1. Certain measurements made on pigeons flying horizontally in a wind-tunnel are described.

2. A method, based on helicopter theory, for calculating the power required to fly at any given speed is explained. Induced, profile and parasite power are calculated separately.

3. It is concluded that the pigeon can fly horizontally without incurring an oxygen debt at speeds from 3 to 16 m./sec. The minimum power speed is 8-9 m./sec. The maximum continuous power output is estimated to be 10.5 W., and the corresponding oxygen consumption about 170 ml./min. The maximum (sprint) power is estimated to be 20.4 W., from observations of vertical climb after take-off.

4. The estimated best lift: drag ratio in horizontal flight is 5.9, giving a range of 11.8 km./g. of fat oxidized for a 400 g. pigeon.

5. It is argued from considerations of structural strength that the early part of the downstroke is used mainly to impart angular velocity to the wing, and that air loads are only developed after most of the angular acceleration has taken place. The tension in the pectoralis insertion may exceed 60% of the breaking tension in fast horizontal flight.

6. The power calculation was repeated for the ruby-throated hummingbird, using published data. Estimated best range is about 900 km./g. of fat oxidized, achieved at 9 m./sec. The corresponding effective lift:drag ratio is 4.1.

7. The variation of power required and power available with size is considered, and the effect on hovering and take-off performance of different birds deduced.

8. Performance estimates for the pigeon and ruby-throated hummingbird are very poor by engineering standards, but consistent with these birds' known abilities, and are in general agreement with estimates of effective lift:drag ratio derived from published data on other species.

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