74 THE BIOLOGY OF BIRDS 



tail is cocked up the air driven backwards strikes it from 

 below and the head region is depressed. It may also be 

 that one side of the tail is spread horizontally while the 

 other side is raised, so that the equilibrium of the body is 

 preserved. A long neck, as in a heron, may function in 

 the same way, and backward extended legs have doubtless 

 their use. There may also be a vertical alteration of the 

 flight by an active shunting of the shoulder-joint forwards, 

 which would raise the head ; or backwards, which would 

 depress the head. 



In his vivid " Animal Life," Professor F. W. Gamble 

 points out that there are among animals four main modes 

 of locomotion which may be compared to the alternatives 

 open to a man in a boat provided with oars and a boathook, 

 (i) He may punt with one of the oars pressed against the 

 floor of the stream. The same sort of leverage, only more 

 complicated, is seen in the walking of a beetle or of a man. 



(2) He may haul with the boathook fixed to brushwood or 

 the low branches of a willow tree. So does the leech draw 

 itself forward when it fixes its muscular mouth ; so does 

 the starfish pull itself up the rock with its many tube-feet. 



(3) He may scull from the stern of the boat with a single 

 oar, displacing the water from side to side. So does the 

 whale with its flukes or the fish with the posterior third of 

 its very muscular body. (4) Finally, he may sit down 

 and row with a pair of oars, as the insect called the water- 

 boatman rows with its legs, or the turtle in the sea, or the 

 penguin which swims with its wings as well as with its legs. 



Now the ordinary flying is plainly of the rowing type of 

 locomotion. The bird has in the down-stroke of the 

 wings to push downwards and backwards a quantity of air 

 heavier than its body — heavier since a short time has to be 

 allowed for the up-stroke. The larger the wing the fewer 

 strokes are required ; the smaller the wing the more rapid 

 the strokes must be. Hilzheimer gives the following 

 figures for strokes per second : sparrow, 13 ; wild duck, 

 9 ; carrion-crow, 3-4 ; stork, 2 ; and pelican, i^. When 

 the bird has got up a certain speed in the air the energy 



