AVES 367 



miles from their nests and 450 miles from shore, and others liberated 

 on the mainland 850 miles away flew directly to the proper island 

 and nests. 



Why Birds Migrate. — One of the commonly accepted theories as 

 to migration is that ages ago the United States and Canada were 

 occupied by non-mi gj-ato7j birds. When the Arctic ice fields moved 

 South during the glacial period, rendering the Northern half of the 

 continent uninhabitable, because of lack of food supply and a low 

 temperature, the birds migrated farther and farther each decade 

 until finally the racial habit of long migrations was established. 

 Another theory is that the birds' original home was in the South and 

 that as the ice retreated northward they sought a less crowded 

 breeding ground, only to return to their winter quarters in the South. 



In some recent work done by W. Rowan of Alberta, Canada, the 

 factor of the physiological impelling force of the developing gonads 

 is emphasized along with the environmental factor provided by 

 varying day lengths.'^ Rowan says: " Migration cannot be looked 

 upon as an act of volition, but as the automatic response to a certain 

 physiological state probably induced by a gonadial hormone. The 

 birds must migrate if physically able to do so." Unquestionably 

 Rowan's point regarding the influence of the gonads is well taken. 

 (See Fish Migration, page 263.) 



We know that birds have a time sense, and that the northerly 

 movement of the robin is correlated with the attainment of a certain 

 mean daily temperature. The average of weather conditions 

 influences the average time of arrival, and the flight of night mi- 

 grants is known to be correlated closely with meteorological condi- 

 tions. Adams found that bird migrants arrive in waves following 

 peculiar types of weather. 



In spite of the evidence collected by many observers, there are 

 those who seem content, like many of the adherents of the " parent- 

 stream " theory in fishes, to dismiss the whole problem by stating 

 that it is probably due to a mysterious sense of direction. 



Others have suggested that a peculiar physical property of the 

 feathers causes the magnetic pole to exert a powerful attractive 

 influence. 



* In his studies on the sexual cycle of the European starling, Bissonnette has shown 

 that increased daily light periods will increase spermatogenic activity. Consult 

 Bissonnette, T. H., 1931 (Jour. Exp. Zool., vol. 58, pp. 281-319), and earlier papers. 



