742 EVOLUTION OF ANIMALS Part V 



taking advantage of upward rushing air currents. A bird hovers, even poises in 

 the air over some object, a hummingbird over a flower, a gull above the water. 

 Birds do other things with their wings; penguins swim with them; geese, broody 

 hens, and fighting cocks strike with them; and birds in general spread them 

 over their eggs and young. 



Special Features of Digestion. Various birds obtain the same kind of food 

 in different ways: an osprey hovers and drops, catching the fish in its claws; 



Flexor 

 muscles 



Fig. 36.14. Mechanism of perching in birds. Leg of 

 crow. The flexor muscles end in tendons that pass behind 

 the joints, beneath a strap of ligaments at the base of the 

 toes, and are distributed to the toes. As a bird flexes its legs 

 and sits on the perch, the flexor muscles contract, pull on 

 the tendons and the toes automatically grip the perch. 

 (Courtesy, Wolcott: Animal Biology, ed. 3. New York, 

 McGraw-Hill Book Co., 1946.) 



herring gulls swoop down and grip it with their bills; a heron stalks or stands 

 motionless till a fish swims by; the kingfisher makes a sudden plunge; penguins 

 swim rapidly under water and grip the fishes in their bills. 



Cormorants, peUcans and others that eat large fishes have small tongues. 

 In sparrows, warblers, small seed- and insect-eaters the tongues are horny, 

 often with inward pointing spines along the sides that catch in the bits of food. 

 The hummingbird has a long cleft tongue with an inrolled membrane on each 

 half which is worked back and forth in the flower to take up nectar. Saliva 

 figures prominently in some birds; in woodpeckers, its stickiness picks up in- 

 sects; chimney swifts use it as glue in nest building. In all birds, digestion and 

 its associated processes are rapid. 



The esophagus is simply a passageway, or a passageway with an expan- 

 sion, the saclike crop, which provides for quick filling when food happens 

 to be plentiful (Fig. 36.15). Chickens, pigeons, and other grain and mis- 



