INTRODUCTION XllI 



those of other analogous animal products and finally 

 determines the possible functions of the preen-gland 

 and its secretions on a firm basis. 



Here, then, is tangible, concrete information on 

 the biochemistry of the preen-gland. Anatomists 

 have provided us with knowledge of its structure 

 and morphology. Combining these sources of in- 

 formation with field observations and considering 

 them together, we can arrive at an understanding 

 of its fundamental nature. The cooperation of the 

 laboratory specialist is essential. 



Field observations only can acquaint us with the 

 facts of bird migration, but their interpretation can- 

 not be undertaken without the use of information 

 accumulated by the trained biologist, facts of struc- 

 ture and of function, of biochemistry, physiology, 

 biophysics and so on. For migration is an example 

 of animal behaviour, and a very fascinating one of 

 extreme complexity. 



It is the purpose of this little volume to examine 

 the subject from this viewpoint — through the eyes 

 of the biologist. It deals particularly with the pos- 

 sible mechanism of migration, or rather with certain 

 aspects of it, those in which the author has taken a 

 somewhat particular interest. If it convinces the 

 ornithologist that the laboratory and microscope 

 may prove real adjuncts to a solution of the prob- 



