124 



Correlation of the Changes Occurring in the 

 Reproductive Organs. 



The development of the reproductive organs shows a some- ^ 

 what similar condition. The testes of the males will be first 

 considered. The first ''vagrants" to arrive in the spring 

 show but shght enlargement (Plate XXI, A). In the first 

 migrants to arrive the increase is more evident (B), En- 

 largement takes place at a fairly uniform rate (C and D) until 

 the arrival of the adult females, when the rapidity of growth 

 is greatly increased (F). The males are now spending their 

 entire time in the marsh and feeding upon an exclusively 

 insectivorous diet. This rapid increase does not occur in the 

 birds which are migrating, but only in the resident birds. 

 (The migrant birds, however, are immature, and therefore 

 may not be strictly comparable.) After this sudden enlarge- 

 ment, growth continues at a uniform rate until the mating 

 period (G, H, I), when the maximum is reached. The rate of 

 growth of the testes of the immature migrant birds is much 

 less than that of the residents, as is well shown in the plate 

 (J, Kj L, M), those of the adult resident bird on May 10 being 

 fully twice the size of those of the immature migrant of the 

 same date. That the sudden enlargement occurring after the 

 arrival of the resident females and during the subsequent days 

 spent in the marsh, and that the difference in degree of de- 

 velopment between the resident and migrating birds, is 

 largely due to the nature of the food, can not, of course, be 

 stated, although there is some ground for so believing. It is 

 probable that the presence of the females serves to stimulate 

 the growth, but it may well be that the animal food is necessary 

 to support it. 



When the migrant females first 'arrive, their ovaries are in 

 a relatively less mature condition than were the organs of 

 the males upon their arrival a month earUer, the ova being of 

 microscopic size (Plate XXI, N). Enlargement is fairly rapid, 

 however, so that by the time the first resident females appear 

 the largest ova are about IJ^ millimeters in diameter (0). 

 Mating occurs while the ova are still immature (P), and the 

 nest is commenced just as the deposition of yellow yolk is 



