I have examined the evidence contained in the valuable and interesting correspondence mentioned above, and have also obtained all the information I could get elsewhere from books of authority and persons who have had special opportunities of observation. At the present day a valuable and novel class of evidence is available--that of observers in aeroplanes. Upon all the material thus obtained I have tried to form an impartial opinion.
There appear to me to be four points to be borne in mind before arriving at any conclusion as to which bird is the quickest in flight, and the maximum speed of which each bird is capable.
Emphasis is laid on the first three of the following points in some of the letters in the correspondence above referred to, but I think that the fourth point is of at least equal importance.
1. Ground speed must be distinguished from air speed.
2. The path of flight must be horizontal.
3. There must be something to show that the bird is flying at its maximum speed.
4. There must be a standard length of flight to which the test is to be applied.
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.”