1. Ground speed must be distinguished from air speed.
It is not generally realised that a bird has two speeds: its speed relative to the ground and its speed relative to the air.
“Ground speed” is “air speed” as influenced by the wind. In a perfectly still atmosphere “ground speed” and “air speed” are the same. To quote one of the writers in the Field of February 11, 1922: “The wind has no effect on the speed at which a bird is capable of driving itself through the air. Take a parallel case, substitute for the bird a caterpillar, and for the atmosphere in which the bird is flying a sheet of paper. The caterpillar can always crawl at a constant speed across the paper, although it is possible to increase the relative speed of a caterpillar to the ground by moving the sheet of paper.”
Or to put the same distinction in the words of another writer in the same number of the Field: “It is the speed of the object over the ground or still water that matters; and if the medium (i.e. air or water) in which the object under discussion is either flying or floating is also in movement, then the pace over the ground will naturally be correspondingly increased or decreased.”
Wind, of course, varies in two ways (1) direction and (2) velocity, and is uniform only at a given height.
The direction of the wind must necessarily be either along the line of flight of the bird, against it, or at an angle with it. In the first of these instances the speed of the bird over the ground is determined merely by adding the velocity of the wind to, and in the second by subtracting it from the air speed of the bird, in the same way as a swimmer’s speed is increased or reduced by the speed of the current. The third case is more complicated, as in this calculation allowance must be made for “drift,” i.e. the tendency of a bird under such circumstances to deviate from its desired course. It is, however, unnecessary to say anything further as to this third case, as the comparison of speeds of various birds can only be made satisfactorily by ascertaining their speeds under identical conditions in horizontal flight.
2. The path of flight must be horizontal.
In the words[6] of Captain C. F. A. Portal, D.S.O.: “If any one has seen a peregrine stooping from 1000 feet at between 150 and 200 miles per hour at a partridge, and has later seen the same peregrine chase the same partridge from a standing start, he will appreciate the importance of considering only level flight. In the first instance, the hawk is nearly 100 miles per hour faster than the quarry, in the second, he can only just overtake it at all. There is no conceivable way of measuring the speed of these downward flights accurately, but no one who has done any hawking will deny that 120 miles per hour is within the power of a great many species. When we come to consider level flight, there is a very different story.”