414 



METAZOAN PHYLA 



Flexor 

 muscles 



number of modifications. These involve modifications of the beak 



(Fig. 300) connected with the kind of food 

 and the manner of securing it. Examples 

 of such are the flat, straining beaks of 

 ducks; the powerful and sharply pointed 

 spearing beaks of herons; the long, slender, 

 probing beaks of snipes and sandpipers; 

 the chisel-like beaks of the woodpeckers; 

 the stout, hooked beaks of gulls, hawks, 

 and owls; the small, slender, and sharply- 

 pointed beaks of the insectivorous birds; 

 and the relatively larger, heavier beaks of 

 the grain-eating song birds. The feet are 

 also modified (Fig. 301) in accordance with 

 the character of the environment and the 

 manner of locomotion. These modifica- 

 tions are illustrated by the lobed feet of 

 diving birds; the webbed feet of swimming 

 birds; the long legs and long toes of wad- 

 ing birds; the possession of two toes in 

 front and two behind by climbing birds; 

 and the long, slender toes and curved claws 

 of perching birds (Fig. 302). Other 

 modifications involve the tail, which is 

 practically absent in some diving and run- 

 ning birds and most highly developed in 

 birds whose powers of flight are greatest. 

 The wings are also variously developed in 

 proportion to the power of flight and are 

 much reduced in all running birds and in 

 some diving birds which do not fly at all. 

 In the diving birds the feet are carried 

 • Fig. 302.-^ Mechanism of perch- ^^^^ ^^ ^j^^ posterior end of the body 



ing in birds. Preparation from a ^ 



crow. The flexor muscles are shown where they serve effectively as propellers 

 ending in tendons which pass behind -^^ underwater Swimming. 



the joint between the tibiotarsus ^ 



and the tarsometatarsus and under 432. Plumage. — ^The plumage of birds 



a bony arch near the upper end of the y^rics Considerably, fitting them for vari- 



tarsometatarsus. Running under a * i n- i i 



ligament at the bases of the toes ous modes of living. Among the flightless 



they are distributed to the individual ^^^.^g ^^^ ^ ^\{\Qh live in dcSCrt regions 



digits. Because of this structure, ^ <- i i j 



when a bird flexes its legs and sits and the feathers of which are slender and 

 upon the perch, the toes grasp the ^^ ^^^^ overlap. The wings of penguins 



perch with a very powerful grip. i r i i • i i 



are covered with feathers which are scale- 

 like. There are also numerous curious modifications of the feathers 

 on the head, wings, and tail. Especially conspicuous among birds 



