BIRDS. 293 



passed from various parts of the surface of the body, and 

 were attached to the skin ; and a beautiful fan-shaped muscle 

 was also spread over the anterior surface of the large air-cell 

 just mentioned. " The use of these muscles appeared to be 

 to produce instantaneous expulsion of the air from these ex- 

 ternal cells, and by thus increasing the specific gravity of the 

 bird, to enable it to descend with the rapidity necessary to the 

 capture of a living prey, while swimming near the surface of 

 the water." 



This is one of those beautiful adaptations of means to an 

 end which Natural History records in every department. 

 " The descent of the Gannet on its prey has been, not inaptly, 

 compared to that of an arrow, the beak of the bird forming 

 the arrow head, and the body and wings the feathered shaft of 

 the weapon — we here have the secret of its heavy fall ; the 

 same machinery restores the buoyancy at the proper moment, 

 and the bird rises with its fish aloft." 



Sloulling. — The plumage of birds is periodically renewed, 

 and the process of this change of feathers is termed " moult- 

 ing." The aspect of the bird, in many instances, changes, not 

 only with age, but also with the season ; the summer dress, as 

 we shall have occasion to mention, is often very unhke that of 

 the winter. The changes in the plumage of bii-ds have been 

 investigated with great care by Mr. Yarrell ; and, in the opm- 

 ion of that able zoologist, the different appearance which it 

 presents may be explained, — 



1st. By the feather itself becoming altered in colour; 



2nd. By the birds obtaining a certain addition of new fea- 



thers, without shedding any of the old ones ; 

 3rd. By an entire or partial moulting, at which old feathers 

 are thrown off, and new ones produced in their 

 places ; and, 

 4th. By the wearing off of the lengthened lighter-coloured 

 tips of the barbs of the feathers on the body, by 

 which the brighter tints of the plumage underneath 

 are exposed. 

 In spring, the change which takes place prior to the pairing 

 season is to be attributed to the first two modes ; and at that 

 time, also, there is a partial moulting of old feathers — a laying 

 aside, as it were, of a portion of the warm garments of winter. 

 The entire moulting is that absolute change of feathers which 

 takes place in autumn. 



