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 
second. 
Sparrow "P Me Aes 
Duck aS: oe poy ee 
Pigeon 8 
Marsh-Harrier Ua le 2 5 
Screech-Owl.. ie a ie 
Buzzard 3 
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, 
