GENERAL HABITS 181 



many birds of exceptional flight endurance are 

 rendered even more buoyant by the presence of 

 curiously modified pneumatic skin, as well as vari- 

 ous air sacs. Vultures, Gannets, and Screamers 

 may be instanced, although it is only fair to state 

 that some anatomists suggest that the principal 

 function of these air sacs is to ventilate the lungs, 

 which in birds are only capable of comparatively 

 limited movement. Even in this case, however, 

 their correlation with flight is obvious. Professor 

 Roy (who contributes a most interesting article 

 on Flight to Professor Newton's Dictionary of 

 Birds, to which we are indebted for many of the 

 following particulars) divides avine Flight into 

 three distinct classes. According to this autho- 

 rity, then, Flight may be performed, firstly, by 

 gliding or skimming ; secondly, by active strokes 

 of the wings ; and, thirdly, by sailing or soaring. 

 In the first place, we must remember that the 

 relation of the wing area to the weight of the 

 bird varies considerably, and that the greater the 

 former relatively is the more powerful the flight 

 will be. An, equally large amount of variation 

 exists in the strength of the wing muscles, this 

 corresponding with their weight. Then, as we 

 have already seen, the shape of the wing is 

 equally subject to much variation, and is largely 



