REPRODUCTIVE ENDOCRINOLOGY IN BIRDS 



1145 



reach the peak of gonadal activity before 

 the females (Benoit, 1956) ; an exception is 

 the turkey in which the female responds to 

 increased photoperiods before the male does 

 (jMargolf, Harper and Callenbach, 1947). 

 The sexual behavior of the male, his sing- 

 ing, and the initiation of nest building, all 

 of which may be dependent on temperature, 

 availability of nest sites, etc., stimulate the 

 females (feenoit, 1956; Lehrman, 1959). 

 Under the influence of this stimulation the 

 female will copulate, and ovulate. She may 

 now lay a more or less predetermined num- 

 ber of eggs (determinate layer) or she may 

 lay eggs until the presence of a sufficient 

 number stimulates incubation behavior and 

 the degeneration of follicles still present 

 in the ovary. 



Destruction of the nest may restart a 

 new cycle; or, in some species, a new cycle 

 may be started after the first hatch has fled 

 the nest. At the end of the summer, in tem- 

 perate zone birds, the gonads regress, and, 

 in migratory species, fat is deposited in 

 intraperitoneal and subcutaneous depots. 

 ]\ligration southward starts under the in- 

 fluence of yet unknown factors. The re- 

 fractory period, which varies in length for 

 different species, is terminated under the 

 influence of short days. In the spring gonadal 

 activity is reinitiated, fat deposition takes 

 place, and migration north starts again, 

 either because of an "inherent rhythm," or 

 under the influence of photostimulation. For 

 many species w^e know nothing about the 

 regulatory factors involved in their migra- 

 tions; therefore, the above generalizations 

 should be taken as tentative even for the 

 best studied species such as those of the 

 genera Junco and Zonotrichia. 



D. REGULATIOX OF THE REPRODUCTIVE 

 CYCLE OF THE FOWL 



For convenience of discussion the repro- 

 ductive cycle of the fowl will be divided into 

 an annual cycle and the laying cycle which 

 encompasses only part of the annual cycle. 



The annual cycle of females resembles 

 the cycle of wdld birds, especially in breeds 

 and strains in which broodiness still occurs. 

 The cycle of the male is virtually absent 

 and males produce sperm all year. Seasonal 

 variations in fertilitv mav occur, but these 



fluctuations may be a reflection of high tem- 

 perature, and they are probably not a re- 

 flection of changes in photoperiodicity. 

 Roosters have been kept for years without 

 molting, although hens normally molt. The 

 hens which show broodiness may become 

 broody several times when not allowed to 

 incubate, and they produce eggs between 

 the broody periods. Legait (1959) studied 

 the annual cycle of Rhode Island Red hens 

 which were allow^ed to incubate the eggs. 

 Her study encompasses all phases of the 

 annual cycle with special attention to the 

 incubation period. Unfortunately, the re- 

 sults seem to be based on 1 or 2 birds for 

 each phase of the cycle, but, if allowance 

 is made for the small numbers, several con- 

 clusions may be deduced from the data. 



1. The diameters of the nuclei of the para- 

 ventricular cells are about 6.24 fx during the 

 annual rest and during the molt; they are 

 somewhat larger, 6.4 to 6.6 /x, during laying, 

 and considerably larger during incubation, 

 7 to 12 /x. 



2. During molting many granvdes of NSM 

 are present in the paraventricular cells, and 

 the posterior lobe also contains NSM in 

 abundance. During incubation, the amount 

 of NSM in the paraventricular cells and 

 posterior pituitary lobe is small. 



3. During laying the percentage of Ao 

 cells in the cephalic lobe of the anterior 

 pituitary is at its maximum and /3-cells are 

 at a minimum. The /?-cells increase during 

 incubation. 



4. In the caudal lobe of the anterior pi- 

 tuitary, S-cells increase during incubation 

 and decrease sharply during the molt. 



5. Adrenal weight is low during the molt 

 and annual rest, but is high during incuba- 

 tion. Legait (1959) show'ed that the diame- 

 ter of the nuclei of the neurosecretory cells 

 and adrenal weights parallel each other. 



6. During incubation and molt, the ovar- 

 ian weight is ciuite small (2 to 3 gm.). 



The annual cycle seems to be regulated 

 largely by light. Recent investigations all 

 indicate, directly or indirectly, that the daily 

 increment in the photoperiod may be more 

 important than the length of photoperiod 



(Sykes, 1956; Morris and Fox, 1958). The 

 experiments carried out by Morris and Fox 



(1958) showed particularly clearly the im- 



