42 S P E E D OF F L I G H T 



other atmospheric conditions. Undoubtedly, the airspeeds of different 

 birds are much more comparable than ground speeds, since variables 

 other than the flying ability of the individuals are reduced to a mini- 

 mum. Air speeds are difficult to obtain, however, since the only means 

 is by the air-speed indicator of airplanes .... The ground speed, how- 

 ever, can be measured more readily, though it is subject to much varia- 

 tion, and the conditions under which observations are made vary so 

 greatly that two records, even for the same species, are seldom entirely 

 comparable. 



"The size and shape of the wings in comparison with the weight of 

 a bird are important factors in determining its speed of flight. The 

 smaller the wings in proportion to the weight, the greater will be the 

 speed required for the bird to maintain its position in the air. More 

 rapid wingbeats against the supporting atmosphere are necessary for a 

 bird with a small wing surface than for one with a wing surface great 

 enough for it to take full advantage of ascending air currents .... 

 Although the weights of the Mallard and Canvas-back are nearly the 

 same, the wing surface of the slow-flying Mallard is about 20 per cent 

 greater than that of the swifter Canvas-back, the latter making up for 

 this small ness of supporting surface by the rapidity of its wingbeats, 

 aided probably also by a reduction of air resistance because of its more 

 streamlined build. It is generally conceded that when two birds of a 

 similar type are once in the air, the heavier bird is the faster. The 

 quicker rise and get-away of the smaller bird, the Teal, for example, 

 gives a false impression of speed. Radclyffe, an experienced falconer, 

 in England, related that he had often flown a peregrine over mixed flocks 

 of Teals and Mallards and flushed the birds simultaneously. In every 

 case, the first bird overhauled by the hawk was a Teal; and in case of a 

 long flight, when every bird was flying for its life, the farther the birds 

 went, the more the Teal lagged behind. Munson reported that while 

 chasing ducks with an airplane, he twice saw bunches of Green-winged 

 Teals that were easily outflown by Canvas-backs in the same flock. 



"It is popularly supposed that birds in migration climb until they 

 find a favoring wind, sometimes to great altitudes. Aviators report that 

 it is exceptional to see any birds more than 5,000 feet above the earth 

 and that few are seen above 3,000 feet. There are, however, records of 

 birds seen at very high altitudes above sea level, but these are mostly in 

 mountainous country where the birds are flying at comparatively short 

 distances above the land. The evidence thus far obtained indicates that 

 the greater part of migration takes place below 3,000 feet above the 

 earth's surface, much of it below 1,000 feet, and that birds prefer to fly 

 below the level of the clouds" (Cooke, 1937). 



The same writer lists some of the recorded speeds of Swans, Geese, 

 and Ducks as follows: 



Whistling Swan, 50-55 m.p.h. air speed, top speed. 



Canada Goose, 60 m.p.h. air speed, chased. 



Canada Goose, 44.3 m.p.h. ground speed, by theodolite. 



