108 



SOME SUGGESTIONS FOR BIRD STUDY. 



toes are turned forward. They walk badly, but can cling 

 readily to walls and such like. The feet of birds are 

 covered with scales and are clawed (Fig. 33). 



Fig. 33. The Feet of Birds. 

 1, Eagle ; 2, Pigeon ; 3, Woodpecker ; 4, Duck. 



Preen gland. On the top of the tail is a small gland, a 

 structure producing the substance with which the bird 

 preens or oils its feathers. It can easily be seen on a 

 plucked bird. 



General Structure. 



There are many other points of interest which might be 

 referred to in the general structure of birds, e.g. the long 

 flexible neck may be regarded as compensatory to the loss 

 of the fore limbs for all purposes save flight ; the large and 

 heavy breast muscles which are required to move the 

 wings in flight ; the presence of numerous air sacs within 

 the body, and in some cases within the bones (humeri) of 

 the wing, greatly contributing to thorough efficiency in 

 respiration ; the high temperature probably related to 

 this ; the arrangement of the heavy organs within the 

 body so that the bird is very stable in the air as well as 

 on land; and numerous skeletal peculiarities, some of 

 which are stated below. 



