66 PERCHING ON ROCKS. 



which both perch and percher are rocked at no mode- 

 rate rate, are all pathways to one race or other of the 

 feathered tribe. Nor, though at first sight such seems 

 to be the case, is that the footing most difficult to be 

 maintained ; for, upon a bending twig or stem, the 

 perch and percher soon acquire the same momentum, 

 swing together, and have no more tendency to sepa- 

 rate than water has to escape from a glass, when that 

 glass is set in one side of a hoop, and the hoop 

 whirled rapidly round on the opposite point. This 

 concert of motion, as we may call it, is general in 

 mechanics, and the application of it is of great service 

 to all birds which feed or repose on perches of the 

 description alluded to. It is the resistance of the 

 bottom and sides of the glass to the centrifugal force 

 of the water, and the resistance of the hoop to the 

 same force in the glass, which keeps these three 

 together, or in concerted motion ; and in like manner 

 when a bird is rocked in the spray, all that it has to 

 do \6 to resist its own centrifugal force by the clutch 

 of the feet. 



But when the bird has to perch on the pinnacles 

 of the rocks, as is the case with the mountain eagles 

 and some of the other mountaineers, and also with 

 several of the more predatory and powerfully-winged 

 sea-birds, it has not the advantage of concerted motion 

 along with its perch, but must either abide at rest, 

 despise the tempest, or drift before its fury. The 

 habitations of those species which sit on the pinnacles 

 of rocks are the very homes of the tempests ; for 

 winds war upon rifted shores and among rugged 

 mountains when the expanse of the sea and the plain 

 are still. Not only this, but these birds live much 

 upon the havoc which the tempest produces among 

 other creatures ; and thus it becomes necessary that 



