622 COLLEGE ZOOLOGY 



little resistance to the air; the wings are placed high up on the 

 trunk to prevent the body from turning over; and the bones are 

 hollow and the body contains air-sacs, which decrease the specific 

 gravity. 



In flying, the tip of the wing describes a figure 8 as it is brought 

 downward and forward and then backward and upward (Fig. 

 508). The wing works on the principle of the inclined plane, 

 and both the down and up strokes propel the bird forward. The 

 body is sustained in the air by the downward strokes, which force 

 it upward. 



A great many birds are able to glide, and a number are fond 

 of sailing or soaring. Birds are able to glide or skim by spread- 

 ing their wings and then moving forward by means of their ac- 



Fig. 508. — Gull flying. (From Headley, after Marey.) 



quired velocity. In soaring, birds do not depend upon acquired 

 velocity, but apparently rely upon favorable air currents. 



The rate of speed at which birds fly varies considerably. The 

 carrier-pigeon in this country maintains an average racing speed 

 of about thirty- five miles per hour. Ninety miles per hour has 

 been recorded for ducks (Forrester) , but this rate is not sustained 

 for any great length of time. During long flights the distances 

 traveled per day are comparatively short, e.g. an albatross is 

 known to have covered over three thousand miles in twelve days 

 or two hundred and fifty miles per day, and a carrier-pigeon 

 flying from Pensacola, Florida, to Fall River, Massachusetts, a 

 distance of over a thousand miles, attained a daily average of 

 seventy-six miles. 



e. Bird Migration 



Formerly birds were supposed to hibernate during the winter 

 in caves, hollow trees, or, in the case of swallows, in the mud at 



