12 THE BIOLOGY OF BIRDS 



bird is plucked. The graceful smooth lines so pleasing in 

 most birds are in great part determined by the overlapping 

 contour feathers of the body, and the naked creature is 

 rather a sorry spectacle. 



§ 2. The Skin 



As if the skin exhausted itself in feather-making there 

 is a striking paucity of skin-glands, for there is usually 

 nothing but the preen-gland at the root of the tail, and even 

 that is absent in ostrich, emeu, cassowary, and a few flying 

 birds such as bustard, some parrots, and some pigeons. 

 The fantail variety has none. The preen-gland is best 

 developed in aquatic birds, where the secretion is believed 

 by many to help in water-proofing. Whereas the mammal's 

 fur is kept sleek, for the most part, automatically, by the 

 secretion of the sebaceous glands at the roots of the hair, 

 though licking the fur is familiar in the cat and some other 

 cases, the feathers of birds are individually cleaned in the 

 bird's bill, and the widespread belief is that this is aided 

 by the oily secretion of the preen-gland. The bird reaches 

 back with its mobile neck and is supposed to get drops of 

 oil from the gland, which it proceeds to distribute over the 

 feathers. 



Mr. Pycraft points out (1910, p. 15) that many birds 

 with a well-developed oil-gland could not possibly take 

 up and spread so much as one drop of this precious fluid. 

 Thus in the Scissor bill (Rhyncops) both jaws are compressed 

 to form a single blade as flat as a paper-knife. Moreover, 

 many birds without an oil-gland, e.g. bustards and ostriches, 

 keep their feathers in as good condition as those birds which 

 possess the organ. Mr. Pycraft suggests that the gland 

 may be of use in producing a characteristic scent, serving 

 for the recognition of kin. On the other hand, birds 

 seem to be on the whole very deficient in the sense of 

 smell. 



The structure of the preen-gland has been very 

 thoroughly studied by P. Paris (1913), who regards it as 



