736 REPRODUCTION AND MIGRATION IN BIRDS 



in the male. The investigations of Polikarpova (1940) with Passer 

 domesticus and Vaugien (1948) on Serinus ccmaria show that com- 

 plete sexual development of the females of these species apparently 

 requires photoactivation of the female, the presence of a photo- 

 activated male, nesting material, and nesting site. Burger (1953) has 

 also emphasized the relative importance of psychic factors in the 

 development of female Sturnus vulgaris. 



EVALUATION OF PHOTOPERIODISM IN CONTROL OF ANNUAL 

 GONADAL CYCLES OF TEMPERATE ZONE SPECIES OF BIRDS 



Because of the limitations established earlier for this discussion 

 and also because of the species which have been investigated experi- 

 mentally, this evaluation will be effected entirely with respect to 

 species which occur at relatively high latitudes. For such species I am 

 definitely of the conviction that the annual vernal increase in day 

 length, together with, in many species, at least, the shorter days of 

 autumn which cause the elimination of refractoriness, constitute the 

 primary timing scheme for the annual gonadal cycles of both sexes. 

 Furthermore, it now appears possible that all the annual cycles in 

 these species are timed, in one way or another, in this way. 



I hasten to emphasize that the photoperiodic timer is the primary 

 timer and that there are many modifying mechanisms, the relative 

 importances of which may vary greatly among different species. 

 Benoit and Assenmacher (1955) have discussed extensively the var- 

 ious factors and mechanisms which may modify the function of the 

 hypothalamo-hypophysial axis. The role of psychic factors in the 

 sexual development of females (Polikarpova, 1940; Vaugien, 1948; 

 Burger, 1949, 1953) has been noted above. Burger (1953), Vaugien 

 (1954a), and Benoit, Assenmacher, and Brard (1956) have found 

 experimentally a variety of psychic effects on testicular development. 



Environmental temperature has often been considered as a possible 

 modifying factor in photoperiodically stimulated gonadal develop- 

 ment. That it is often, at least, relatively unimportant is evident from 

 the responses obtained in zero and subzero temperatures (Rowan, 

 1925, 1926; Bissonnette, 1937; Schlidmacher, 1938a,b; Polikarpova, 

 1940). However, more detailed quantitative investigations of the 



