xii INTRODUCTION 



with the existence of a preen-gland just above the 

 root of the tail in most species. It, Hke migration 

 itself, has received a great deal of attention from the 

 field worker, and speculations as to its possible uses 

 are frequent. One author maintains that it oils and 

 waterproofs the plumage: another denies it. Yet 

 another, by the use of blotting- and tissue-paper, 

 attempts to prove or disprove the presence of oil 

 and so on. The accumulated observations as to 

 how various species make use of the gland are of 

 interest and value, but the suggestions as to its 

 functions are merely speculations. 



The preen-gland has also received attention from 

 an entirely different source. The papers published 

 have never appeared in ornithological journals. In 

 all probability they have not even been reviewed 

 there, leaving the field man quite unaware of their 

 existence. They are the product of the laboratory 

 and embody the findings of the trained biochemist. 

 The test applied for fats is not blotting-paper but 

 the most precise chemical analysis. Moreover the 

 biochemist, not content with the more or less casual 

 nature of simple observation, systematically extir- 

 pates the gland from birds kept under controlled 

 conditions of food and lighting, records by means 

 of scientific methods the precise effects on plumage, 

 nutrition and vitality, compares his analyses with 



