Natural History 405 



the bird being thus more nearly equivalent to a helicopter than to 

 an aeroplane. 



Speed and Altitude 



The aviators of to-day compete to establish records for speed, 

 for endurance, and for altitude. How do birds stand in these re- 

 spects? As regards speed, in the first place one must remember 

 the difference between "ground speed" and "air speed." Both 

 the aeroplane and the bird can, for a certain expenditure of power, 

 attain a certain velocity in the body of air in which they are, but 

 the velocity as measured from the ground may be a very different 

 thing. Thus an aeroplane travelling at 100 miles per hour in a 

 20 miles per hour wind may seem from the ground to be going at 

 120 miles or at 80 miles per hour, accordingly as it flies with or 

 against the air-stream ; so also, of course, with the bird. All our 

 speed records of birds, except a few made from aeroplanes, are 

 necessarily in terms of ground speed, and in many cases the par- 

 ticulars necessary for a wind correction are unhappily wanting. 



What are some of the actual figures? The available evidence 

 has recently been summarised by Colonel Meinertzhagen, with 

 special reference to speed during migration; he concludes that a 

 bird has an ordinary pace, which is the one used in migratory 

 flight, and an accelerated pace of which it is capable for a short 

 distance under stress of danger or in other special circumstances. 

 Here are some of his figures: carrier-pigeons, 30-30 miles per 

 hour (over 60 has been recorded, but possibly only with a strong 

 favourable wind) ; crows, 31-45; small song-birds, 20-37; star- 

 lings, 38-49; ducks, 44-59; he also quotes the case of a flock of 

 swifts flying at 6,000 feet above Mosul, in Mesopotamia, which in 

 their ordinary flight easily outpaced the observer's aeroplane when 

 it was doing 68 miles per hour. The air speed of this astonishing 

 flyer is, when accelerated, probably well over 100 miles an hour. 



As regards altitude, it seems that although birds have occa- 

 sionally been recorded as high as 15,000 feet, they are indeed 



