58 THE RIDDLE OF MIGRATION 



is normally but little sunlight as, for instance, the 

 islands of Behring Sea, notoriously sunless, which 

 have been inhabited for centuries by tribes of 

 Eskimos. The sebaceous glands of mammalian 

 skin, on the other hand, contain a chemical com- 

 pound known as ergosterol. When this is exposed 

 to ultra-violet rays, whether artificial or natural, 

 vitamin D is elaborated. It is resorbed by the 

 skin and the animal is thus able to obtain a supply 

 regardless of the kind of food it eats. Birds poSvSess 

 no sebaceous glands but in the single preen-gland, 

 situated above the tail and present in most birds, 

 they have a close equivalent. Analyses of preen- 

 glands of various species in the Department of Bio- 

 chemistry in the University of Alberta have demon- 

 strated the presence of ergosterol, and it is present 

 also in the fats extracted from the feathers. When 

 a bird preens, and therefore, as long as it is exposed 

 to sunlight, it will be absorbing small doses of vita- 

 min D as it passes the feathers through its bill. 

 Much more detailed studies carried out more re- 

 cently at McGill University have proved similar 

 in their findings and demonstrated a very close 

 relationship between the preen-gland and avian 

 nutrition. 



Vitamin D deficiency in young animals produces 

 rickets, a common disease of children in the northern 



