760 



REPRODUCTION AND MIGRATION IN BIRDS 



15 



^ lU 



45" N. 



molt 



testes 

 recrud.? 



1 \ 



\ / ■' 



Solstice 



I 



Equinox 



Solstice 

 I 



Equinox 

 I 



Solstice 



I 



15 



\h 



12 



Fig. 1. Approximate annual cycles of day length (sunrise-sunset) to 

 which bobolinks {Dolichonyx oryzivorus) are exposed. Broken lines 

 indicate northward and southward migrations. Precise data are lacking 

 for onset of northward migration and also for arrival in the Southern 

 Hemisphere. The duration of the southward migration appears to be 

 rather variable, especially in the southern United States; the probable 

 extremes are indicated. 



The "wintering" grounds lie in South America, mainly between 

 15° and 25° S. Lat. Once across the equator, the bobolinks are 

 exposed to days more than 12 hr long, increasing up to about 14 hr 

 at the time of the southern solstice; after the solstice, days again 

 decrease to about 12 hr at the March equinox. I do not know when 

 the northward migration begins; presumably this occurs sometime 

 during March. Once again north of the equator, the birds are exposed 

 to very rapidly lengthening days, not only because day length now 

 is increasing everywhere in the Northern Hemisphere, but also because 

 the rate of this increase increases rapidly in the higher latitudes 

 toward which the birds are moving. 



SECONDARY SEX CHARACTERISTICS 



Bobolinks are seasonally sexually dimorphic. The male nuptial 

 plumage develops after a molt in the Southern Hemisphere and 



