6 Introduction 



typical rod shape. He therefore concludes that "the metallic colors of these 

 feathers are probably thin-plate interference colors or Newton's-rings effects, 

 which are produced when spherical pigment granules come in contact with the 

 outer transparent layer." The whole subject of coloration is treated at length 

 by Newton in his " Dictionary of Birds," to which the reader is referred for 

 fuller information. 



/Pterylosis. It is a fact of common observation that in the hair covering of 

 ^certain mammals, such as the horse, dog, cat, etc., the hairs are set as closely 

 together as practically possible, forming a continuous covering. From the fact 

 that in most birds, the entire body, except the beak and feet, is ordinarily covered 

 by the feathers, it might be inferred that they are as continuously and evenly dis- 



FIGS. 3 and 4. Pterylosis of Nighthawk. 



tributed over the body as are the hairs of mammals. This condition is far from 

 true, however, as may be readily demonstrated by plucking the feathers from the 

 body of any common bird, when it will be seen that the feathers ar<* only borne 

 on certain definite areas or tracts, being ordinarily spread out so as to more or less 

 completely cover the body. This peculiarity was noticed and especially empha- 

 sized by Nitzsch, a celebrated German ornithologist, some sixty years ago. 



The feather areas he called " feather-forests," or pteryla, and the naked spaces 

 apteria, from the Greek, signifying " without feathers." The description of the 

 feather distribution in birds is called pterylosis or pterylography. The most 

 important of the feather tracts are as follows: A spinal tract which runs down 

 the backbone from the nape of the neck to the tail; a ventral tract, which "runs 

 from the throat down the front of the neck, and dividing at its base, passes down 

 on each side of the breast and abdomen to the inner side of the thighs quite to 

 the end of the body"; the humeral tracts, a pair of tracts running across the upper 



