GREEN WOODPECKER. 



135 



tlie bird, is the small size of the keel of the breast-bone. 

 Moderate powers of flight, sufficient to transport the bird 

 from tree to tree, are all that it seems to require ; large pec- 

 toral muscles with a deep keel to the breast-bone would to 

 this bird be an inconvenience. The advantage of a narrow 

 shallow keel is immediately apparent, on looking at a repre- 

 sentation of the skeleton in a climbing position : the low keel 

 allowing the bird to place its body close to the tree, brings its 

 centre of gravity in a perpendicular line before the points of 

 support, and thus materially diminishes the labour of, and 

 the strain upon, the muscles of the legs and thighs. The 

 descending position of the bones of the tail indicate the mode 

 by which the stiff points of the tail-feathers are brought into 

 contact with the surface of the bark of the tree to form an 

 accessory prop. 



