1144 



SUBMAMMALIAN VERTEBRATES 



the inability to perch prevented the secre- 

 tion of gonadotrophins. Ficken, van Tien- 

 hoven, Ficken and Sibley (1960) found that 

 mirrors in cages of pairs of budgerigars de- 

 layed ovulations but did not affect spermat- 

 ogenesis. In this respect the budgerigars may 

 differ from pigeons in which a mirror was 

 stimulatory when one female was in a cage. 

 We were not able to find evidence about the 

 effect of mirrors in cages with pairs of pi- 

 geons. 



The observations and experiments re- 

 viewed indicate that light may be a very 

 important factor in determining the onset of 

 gametogenesis, of migration, and of fat de- 

 position in certain migratory birds. How- 

 ever, other factors (e.g., rain) may modify 

 the onset of breeding, and in some species 

 these factors rather than light seem to be 

 most important. Such species can then breed 

 at more irregular intervals when conditions 

 are favorable, e.g., after rainfall. Some trop- 

 ical species such as the sooty terns, Sterna 

 fuscata, breed every 9.6 months (Chapin, 

 1954) , apparently stimulated, not by outside 

 factors, but by an inherent rhythm (Mar- 

 shall, 1959). The multiplicity of factors de- 

 termining breeding cycles of avian species 

 makes it unlikely that any hypothesis will 

 be useful when it takes into account only 

 one of these factors such as light. 



The discussion of the initiation of the 

 breeding cycle should be followed by a dis- 

 cussion of the factors which terminate the 

 cycle, but relatively little is known. In those 

 species in which the male helps in incubation 

 or in building the nest or stays with the fe- 

 male while she is incubating, the release of 

 prolactin in response to visual and emotional 

 stimuli may cause the regression of the tes- 

 tes. Some evidence for this is found in the 

 experiments by Patel (1936) in which the 

 sight of an incubating mate caused crop 

 gland development in the male. Experi- 

 mentally, crop gland development can be 

 induced by prolactin, and prolactin is known 

 to cause regression of the gonads. In polyg- 

 amous species the regression of the testes 

 does not seem to occur until rather late in 

 the summer when decreasing daylight may 

 be a causative factor. 



For the females a distinction should be 

 made between deterininate and indetermi- 



nate layers. Tlie former lay a definite num- 

 ber of eggs per clutch whether or not eggs 

 are removed. Examples are brant, snow 

 geese (Barry, 1960), budgerigars, Agapornis 

 roseicollis, A. taranta, A. fischeri. From the 

 rather scanty data available, it seems that 

 two mechanisms are involved in making 

 these birds determinate layers. In brant and 

 snow geese only a limited number of follicles 

 (5 to 6) reach ovulatory size, and these are 

 ovulated when conditions are appropriate 

 (Barry, I960). Ovulation, or atresia of the 

 largest follicle, permits the next follicle to 

 reach maturity so that it can be ovulated if 

 conditions are right. In Agapornis and in 

 budgerigars more follicles mature than are 

 necessary for the clutch, but after the nor- 

 mal number of eggs (5 to 6) are laid, a gen- 

 eral atresia of the other follicles occurs. This 

 information on Agapornis and budgerigars 

 was kindly given to me by Dr. W. C. Dilger. 

 The factors causing this atresia are un- 

 known. 



Indeterminate egg laying has been ob- 

 served in many species (Lehrman 1959, and 

 his chapter in this book). In brief, the evi- 

 dence suggests that the female starts to in- 

 cubate after a certain number of eggs are 

 present in the nest, whether laid by herself 

 or placed in the nest by others. After this in- 

 cubation starts (and prolactin is released?) 

 degeneration of the follicles takes place and 

 no further ovulations occur. Lehrman (1959) 

 cites Poulsen's work in which laying could 

 be repressed in pigeons if 2 eggs were placed 

 in the empty nest. As pigeons are determi- 

 nate layers, he could not obtain more than 

 2 eggs by removal of 1 or 2 eggs as they 

 were laid. This whole field of investigations 

 on the physiologic mechanisms involved in 

 deterininate and indeterminate layers is vir- 

 tually unexplored, in spite of many field ob- 

 servations which suggest what mechanisms 

 may be involved. 



The following may serve to summarize 

 the present concepts of the regulation of 

 the breeding cycle of seasonally reproduc- 

 ing birds. 



Various stimuli or combinations of stim- 

 uli, such as light, rainfall, availability of 

 food and nesting material, and vocalization 

 by other birds in the flock, may initiate 

 gametogenesis. In many species the males 



