44 INTRODUCTION. 



ly. A person vaguely observing the flight of a bird might sup- 

 pose that the wing is stretched out stiif, but on the contrary, it 

 undergoes an undulatory motion during its descent. Many birds, 

 however, as the Golden Eagle and the Raven, having given 

 themselves a sufficient impetus by repeated vigorous exertions, 

 can glide with extended wings over a considerable space. This 

 mode of flying has by some been called sailing, but the nomen- 

 clature of flight is at present in a most barbarous condition, being 

 confined to the general terms, flying, gliding, sailing, hovering, 

 skimming, and a few others. Birds in flying always beat their 

 wings synchronously, never alternately, even in executing rapid 

 turns, w^hich are effected by the contraction of one wing and the 

 extension of the other, aided by the tail. 



The modifications of flight are numerous, different fami- 

 lies of birds, and sometimes even different species of a genus, 

 exhibiting marked peculiarities with respect to it. Thus 

 Grouse and Partridges, which have short, rounded, concave 

 wings, fly by quickly repeated strokes ; while Gulls and Terns, 

 which have long, pointed, straight wings, fly by slowly repeated 

 strokes, in a light and buoyant manner. The modifications in 

 the form of the wing are not less numerous than those in the 

 mode of flying ; but as both will be amply illustrated in the 

 following pages, it is unnecessary to treat of them here. 



As connected with flight, however, it may be proper to say a 

 few words respecting the tail, and the muscular apparatus by 

 which it is moved. 



Fig. 5. represents, as seen from above, the tail of a Thrush, 

 Tardus musims, from which have been removed all the smaller 

 feathers and the skin. A portion of the sacrum and pelvis, a ; 

 the large tail-feathers or quills, b, h. These feathers have their 

 basis supported by the last coccygeal bone, and firmly bound to- 

 gether by a strong ligamentous band, composed of interlaced 

 fibres. On its upper surface rests the uropygial gland, celebrated 

 by the field and closet naturalists, being one of the few points of 

 the structure of a bird accessible to them, and containing a quan- 

 tity of oily matter mixed with an aqueous fluid ; while on its 

 lower surface is a layer of cellular tissue containing a similar 

 substance. Both are apparently destined for nourishing the 



