FACTORS AND STIMULI 215 



winter. However, due to the conditions under which the birds used 

 were kept, the results may not be applicable to unsheltered wild birds. 



"It is possible that the thyroids of this species do not respond to 

 cold weather in the same manner as do those of the non-migratory 

 forms and that this is significantly or causally related to the migra- 

 tory instinct. This migratory, cold-avoiding species must do more 

 work — must produce more heat — to maintain itself at lower tempera- 

 tures than is required in either of two related non-migratory species 

 of dove already studied." 



Present Status. A synthesis of the results of the preceding re- 

 searches serves as a touchstone to test the various hypotheses brought 

 forward in the past, especially those based upon gonad hormones or 

 the length of the day. As early as 1890, Giitke maintained that it 

 could not be the reproductive instinct that prompts the spring migra- 

 tion, since many species do not breed in the first, second, or even 

 third year, but migrate to their homes like their fully mature con- 

 geners. Marshall (1910), though favoring the gonad hypothesis, ad- 

 mitted the grave objection presented by the behavior of juveniles, and 

 Kendeigh has recently emphasized the decisive nature of this. The 

 evidence supplied by Bergtold (1926) to the effect that migratory 

 males have heavier gonads than those of non-migratory species is too 

 inconsistent to be significant, since 6 of the 11 non-migratory forms 

 exhibit a 200-fold or greater increase, while 5 of the 13 migrants are 

 below this level and 2 others but little above. However, the notable 

 experiment of Rowan with normal and castrated crows appears to 

 remove gonad excitation from the list of possible causes, and all the 

 more definitely because of the conclusions drawn from his earlier 

 studies. When these are taken in conjunction with the absence of 

 such stimulus in migrating juveniles, gonad control seems eliminated 

 and to be at most only a concomitant. 



As to length of day, Schafer, Groebbels, Rowan, Kendeigh, and 

 others are evidently justified in interpreting this primarily as a mat- 

 ter of time for greater activity and food gathering. With respect to 

 the question of radiation. Rowan feels that exercise and not ultra- 

 violet is concerned, while Bissonette (1930, a, b, 1933, 1937) believes 

 that it is light alone and that the male gonads are conditioned by 

 the daily period, intensity, and wave length of the light. Rowan has 

 disproved the influence of the gonads on fall migration, but has not 

 dealt experimentally with their effect in the spring movement. Allard 

 (1928) from the vantage ground of a pioneer in the field of photo- 

 periodism, inclines to reserve judgment as to the duration factor. 

 Finally, no one seems to have pointed out in this connection the 



