ALTITUDE AND SPEED OF FLIGHT 



But mere keenness of vision fails to account for 

 the facts of migration, although, of course, it 

 plays a more or less important part in the matter 

 of way-finding. 



As Col. Meinertzhagen remarks : ' ' How can 

 sight guide birds on night journeys, when on dark 

 nights birds fly but a few feet above the surface 

 of the earth or sea?" 



Amongst the main conclusions reached by him 

 are the following : — Birds need not, for the pur- 

 pose of migration ascend much beyond 5,000 feet 

 above the level of the earth, nor indeed do they. 

 Birds met with above this level are the exceptions 

 and not the rule. In normal conditions, different 

 species travel at different altitudes, some very 

 low and some invariably high, but during 

 abnormal weather-storm and fog all birds are 

 apt to fly low. The oft- voiced belief that greater 

 altitude makes flight easier for a bird is not borne 

 out by fact. The experience of the Royal Air 

 Force is that as altitude increases the machine 

 has greater difficulty in maintaining height and 

 speed. There is no reason to think that this 

 does not apply equally to birds. The velocity 

 and direction of wind have little effect on altitude. 



Speed. 

 In regard to the relative rates of speed at which 

 birds travel great differences of opinion have 

 existed. In Gatke's estimate the speed 

 attained is far greater than subsequent au- 

 thorities are prepared to admit. Gatke believed 

 that a grey crow, travelling across Heligoland, 

 would pass the coast-line of the island at 8 a.m. 

 and arrive on the Lincolnshire coast about 11 a.m., 

 thus covering the 360 miles at the rate of 120 

 miles per hour. A further estimate satisfied 



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