50 



THE FLIGHT OF BIRDS 



unless they have, by some means other than a 

 jump, attained some elevation. Hence the difficulty 

 they have in starting from level ground. But even 

 when this full sweep is possible big birds are still 

 only capable of ascending with a gentle incline, and 

 this is a fact that we must try to account for. 



Loss of Altitude between Wing-Strokes. 



To my thinking the explanation is this : since 

 the big bird necessarily takes a longer stroke and 

 requires more time to raise his wings for a fresh 

 downward beat, he must inevitably lose more 

 altitude between wing-strokes than the small bird. 

 Professor Marey gives the rate of stroke for a 

 number of birds as registered by means of scientific 

 apparatus.* 



Strokes per 



In the case of a Sparrow it is evident that there 

 is no time for a drop between the strokes, though 

 compared with those of most insects even a 

 Sparrow's wings move slowly and heavily. The 

 leisurely working of a Heron's wings is familiar 

 to everyone who ever takes the trouble to observe 

 birds ; usually he flaps along with only 130 strokes 

 a minute or even slightly less. The Marsh-Harrier 



* See his Vol des Oiseaux, p. 100 k 



