Endocrine Mechanisms 767 



gonads does not appear to abolish migrational behavior.^^ Furthermore many 

 birds migrate while they are still sexually immature. ^''^'* Migrant juncos re- 

 tained as long as two months after their normal time of migration would 

 still move northward on release, although their gonads were already in breed- 

 ing condition. ^^'^ Birds retained in their wintering grounds during the sum- 

 mer, and thus prevented from spring migrating and breeding, nonetheless 

 underwent the typical gonadal regression and fat deposition observed in 

 birds preparing for the annual fall migration. 



Examination of the pituitaries of juncos showing increase in testis size, 

 and of birds later during breeding, revealed cytological evidence of much 

 more active secretion of this gland than during the winter when testis size 

 was minimal. Furthermore, injections of antuitrin-G containing several an- 

 terior lobe principles will induce a condition— increased gonad size and 

 heavy fat deposition— which resembles closely those characteristics in a bird 

 ready to migrate. These observations strongly suggest that the pituitary is 

 an important conditioning agent for migration. ^^'•* 



Within many species of birds, including juncos, it is possible to differen- 

 tiate two types of individuals. Some individuals are normal migrants, and 

 others are non-migrants or residents. These two types can sometimes be 

 clearly differentiated into morphologically different subspecies or varieties, 

 whereas again they may comprise quite similar individuals. An individual 

 bird may belong to both types at different times during its lifetime. There 

 appears to be good evidence that similar treatment of "residents" and "mi- 

 grants" with increased light periods will result in gonadal recrudescence in 

 both instances, yet only the migrants will exhibit fat deposition and subse- 

 quent migration. 



Such observations as these lead to the concept that an inherent potentiality 

 to migrate exists in certain birds, but that the behavior patterns which init- 

 iate and carry out the migratory flight are activated or influenced by season- 

 ally varying external factors, principally temperature,^''- ^" (see p. 371) and 

 changing day lengths. These factors appear to operate initially by way of the 

 anterior lobe of the pituitary ~^' Hormones from this organ then gradually alter 

 the physiological state of the bird, one or more of these Jatter alterations prob- 

 ably serving as the immediate stimulus to migration. The data suggest that the 

 fundamental alteration in physiological state involved here is the deposition 

 of fat which would provide the energy for the migrational flight. 



GENERAL CONSIDERATIONS 



In any comparative survey of endocrine mechanisms one is impressed by 

 the fact that, in all those animal groups in which endocrine systems have 

 differentiated, the same general types of functions are being subserved by 

 them. In the two very widely separated animal groups, the Arthropoda and 

 the Vertebrata, which are considered by most zoologists to possess no com- 

 mon ancestry short of relatively primitive forms of life, we see the same 

 general distribution of coordinatory functions between nervous and en- 

 docrine systems. In both groups growth, differentiation, reproduction, meta- 

 bolism, and pigment cells are influenced by hormones. In brief, the endo- 

 crine system seems to influence primarily those functions in which the 

 time to induce a response is long (as in those activities involving growth 



