REGULARITY OF RETURN 217 



breeding season is the insuring of an adequate food supply for rear- 

 ing the young in the nest. Wlien the young leave the nests, the 

 demand for food within the community is not necessarily enhanced, 

 as the juveniles scatter, abandon the territories, and may leave the 

 region (Williams, 1936). Such an early departure may occur when 

 the production of plant and insect foodstuffs is at or near its height. 

 In general, summer or autumn migration begins in a period of waning 

 days and usually falling temperatures. There is a gradual tendency 

 in the direction of a necessity for more activity to secure the same 

 amount of food with the accompanying tendency toward a changed 

 metabolic state. It is not improbable that a diminishing fund of vita- 

 mins may have some share in the general outcome. 



Some of the major features of the biochemistry of the metabolism 

 that leads up to the migration emerge from the studies of Riddle and 

 his associates, and still others may be inferred from them. Of basic 

 importance in this connection is the seasonal cycle of blood sugar, its 

 relation to food and activity, and its fluctuations with temperature, 

 falling with cold and rising with heat. It tends to drop in autumn 

 with lower temperatures and increased thyroid function, and it rises 

 in the spring in response to warmer weather and especially to the 

 greater activity of the suprarenal glands. These glands would appear 

 to play a leading part in metabolism at the time of the spring migra- 

 tion through their varied effects. Adrenalin not only calls forth 

 glycogen from the liver, with an ensuing rise in metabolism and heat 

 production, but it may also constrict the capillaries and prevent the 

 release of heat, while at the same time acting directly upon the 

 nervous system. Riddle's studies have shown that the suprarenals 

 enlarge and that blood sugar and calcium increase at the time of 

 ovulation in pigeons, which may well represent the causal sequence to- 

 gether with pituitary control. If it be shown that this hypertrophy 

 is associated with the storage of vitamin E in the glands (Szent- 

 Gyorgyi, 1933), then the way is opened for relating the entire process 

 directly to the green food supply of spring (cf. Heape, 1931). In 

 this biochemical pattern, the antagonism between the thyroid and 

 the suprarenals, and possibly the gonads as well, would seem to bear 

 some part, as well as that between adrenalin and the oxidation pro- 

 moters, such as insulin. 



Regularity of Return. Much emphasis has been laid upon the 

 regularity of appearance of migratory species, especially in spring and 

 particularly by proponents of the hypothesis based upon day length 

 and gonad hormones (cf. Wetmore, 1930:73; Schafer, 1907). With 



