GENERAL HABITS 183 



backwards, by retracting it, or (in a short-necked 

 bird) by moving forwards or backwards the ex- 

 tended wings, or opening or closing the tail. 

 Change of direction upwards or downwards can 

 thus be obtained by moving backwards or for- 

 wards the centre of gravity, and this without 

 changing the angle of the under surface of the 

 wings and tail with the general direction of the 

 flight. All this involves but little muscular exer- 

 tion, and a similarly small amount of energy. A 

 turn to the right or left is made by changing the 

 centre of gravity to one or the other side of its 

 centre of suspension by the expanded wings, 

 either by a turn of the head, the course following 

 the same direction, by partly bending one wing, 

 or raising one side of the expanded tail, which 

 also causes a turn in the same direction. 



Flight by active strokes of the wings is the 

 most ordinary method adopted by birds in pass- 

 ing from place to place. As Professor Roy 

 points out, it is much easier for a bird to sustain 

 itself in the air by active flight, if it have a certain 

 initial relative velocity, than when it begins to fly 

 from a point of relative rest. Most large birds, 

 especially, therefore seek to rise from the ground 

 with their heads to wind. In the absence of wind 

 (and in some species this condition is immaterial) 



