BIOPHYSICS OF BIRD FLIGHT — RASPET 



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WATER TEMPERATURE MINUS AIR TEMPERATURE IN "C 



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Figure 10. — Modes of soaring of herring gulls, based on temperature increment and wing 



speed. 



Just how tlie bird measures the vertical velocity and just how it de- 

 termines which way to turn in order to stay in the upcurrent are 

 questions which we cannot presently answer. 



Another source of energy for soaring is that provided by thermal 

 upcurrents. These exist both in hilly and in flat country. A very 

 thorough exposition of the nature of birds soaring on thermal up- 

 currents is given by Huffaker [10]. Not only did Huffaker in 1897 

 clearly describe the bird's thermal soaring but he also indicated that 

 there is good reason to believe that birds have some means for detecting 

 thermal upcurrents at a distance, for they often head directly for a 

 given area and begin circling. They inevitably gain altitude. 



Some years ago I speculated that the bird must measure in some way 

 the temperature gradient in the horizontal plane and that from this 

 gradient it is able to determine the direction toward the warm up- 

 current core. An attempt to do this in a sailplane merely proved that 

 we know too little about the nature of thermal upcurrents to be able 

 to devise instruments for prospecting for the thermal upcurrents [11]. 



Another form of upcurrent, still a thermal upcurrent but over 

 water instead of land, was beautifully studied by Woodcock [12], 

 using herring gulls as his indicators of the nature of the upcurrent. 

 In figure 10 is shown a plot taken from Woodcock's paper, which 

 delineates the type of thermal upcurrent, a columnar or cylindrical 

 vortex with axis horizontal. This research is a clear example of the 



