BRITISH BIRDS OF FASTEST FLIGHT 25 
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 w^hich 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 cater- 
pillar 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." 
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