640 A CENTURY OF PROGRESS IN THE NATURAL SCIENCES 



The fundamental cycle which periodically places the bird in the "disposition 

 to migrate" is probably the result of the cyclic function of the anterior lobe of 

 the pituitary gland. This cycle could be, and probably is, the result of periodic 

 change in stimulation by periodic external environmental factors such as day- 

 length. When a certain threshold of stimulation is reached, the bird is stimu- 

 lated to migrate. The act of migration, that is, the actual movement through 

 space on a remarkably exact time schedule, is the result of a complex inherited 

 behavior pattern stereotyped in the nervous and endocrine systems. 



The problem of orientation and navigation during the migratory flight posed 

 a still more difficult problem. The classical experiments of J. B. Watson and 

 K. S. Lashley (1915) had proved that nesting birds could find their way "home" 

 over long, unfamiliar routes. Further experiments on homing have been car- 

 ried out by Riippel in Germany, Lockley in England, and Griffin in the United 

 States. All tended to confirm the fact of homing ability in birds but failed to 

 yield unquestionable proof of the method of orientation. The importance of 

 landmarks in the homing of carrier pigeons was established by several workers, 

 including the Heinroths (1941). 



The hypotheses presented by Ising (1945) and Yeagley (1947), postulated 

 that orientation could be achieved by detection of variations in the fields of force 

 resulting from the earth's rotation (Coriolis force) were vigorously attacked 

 by both physicists and biologists (see Odum, 1948). 



The most promising development in the field of orientation research is the 

 work of Gustav Kramer (1949, 1950), who has successfully demonstrated that 

 the sun is utilized in orientation at least by certain diurnal migrants. Kramer 

 constructed a round cage having six equally spaced windows. Each window was 

 equipped with a hinged shutter upon which a mirror was mounted. By manipu- 

 lation of the shutters the angle of the sun's rays entering the cage could be modi- 

 fied. With the shutters wide open a spring migrant European starling {Sturnus 

 vulgaris) made repeated attempts to fly toward the northwest, the normal di- 

 rection for the spring migration. AVhen the mirrors were placed so as to deflect 

 the direction of the incidental light by 90° the captive bird changed the direc- 

 tion of its flight in accordance with the direction of the light. 



There still remains the problem of orientation by nocturnal migrants but 

 Kramer's experiment will certainly direct further research along profitable 

 pathways. 



The paper by Drost (1950) provides a review of much of the recent work 

 on bird migration. 



Bird Behavior 



The necessity of objectivity as a component of the scientific method is unde- 

 niable. It is equally certain that no field of endeavor has had a more difficult 

 time incorporating the objective viewpoint into its investigations than that of 

 animal behavior. Not until it emancipated itself from the burden of anthropo- 

 morphism was it able to attack its problems with any measure of success. 



The viewpoints of Christian Ludwig Brehm (b. 1787, d. 1864) and his son 

 Alfred Edmund Brehm (b. 1829, d. 1884) dominated the thinking on bird be- 

 havior during the mid-nineteenth century. The younger Brehm's two great 

 works, Das Lehen der Vogel (1861) and IHustrirtes Thierlehen (1864-1869), 



