When the aviators came home after the first World War there appeared in a number of the world's bird journals accounts of the flyers' experiences with birds. Notable among these were the contributions made towards increasing our knowledge of the speeds at which birds can fly. Already the war just ended has produced a good crop of aviators' bird stories, and, as time goes on, there will no doubt be added to the ornithological records some further examples of air plane-speedometer-timed bird speeds. There are a number of factors in the speed of birds that the bird lover, untrained in the lore of aerodynamics, may overlook. A fairly common error derives from the fact that a large number of speed records are based upon pacing by motorcar or motorcycle. But such records, by themselves, are largely worthless from a scientific point of view, because speed on the ground and speed in the air are quite different.
What are the factors involved in timing the speed of birds? First there is the speed and direction of the wind at the height and in the immediate vicinity of the bird. Second one must consider the direction, including gain or loss of height, in which the bird to be timed is flying. Next is the distance covered by the bird during the test, and whether the bird keeps a fairly even pace throughout the course over which it is timed. Finally, there is the type of flight and the physical condition of the bird as it affects its flight. Although interesting and, within limits, valuable records can be obtained without all these condition being fulfilled, it is neglect of these factors that has been responsible for many of the apparently contradictory figures published in the past.
A brief explanation will show how misleading figures can be obtained when these conditions are ignored. A wind blowing in the same direction as a bird is flying virtually adds its own speed to that of the bird, and a contrary wind has the reverse effect Hence bird speeds timed from the ground, whether paced by car, or timed over a course by stop-watch are inaccurate unless accompanied by some information concerning the wind conditions at the altitude at which the bird being timed is flying. Broun and Goodwin, in their interesting paper in Auk for October, 1943, say of 167 birds timed over a course, only one record was uncomplicated by wind factor and thus was of true air-speed. This was of a sharp-shinned hawk, which, flapping continuously travelled over the course at 34 m.p.h.
During the same tests an osprey, with a ground speed of 80 m.p.h., was actually doing little more than "making use of a very strong thermal (rising current of warm air), so the bird was in reality in steep diving flight without losing altitude."
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