22 THE PHYSIOLOGY OF REPRODUCTION 



so soon, and it is well known that the time of greatest production 

 is earlier if the birds are given shelter. 1 



With the approach of the breeding season the genital organs 

 grow enormously until the whole oviduct reaches a state of hyper- 

 trophic turgescence. Gadow states that in the common fowl the 

 oviduct at the period of rest is only six or seven inches long and 

 scarcely a line wide, but that at the time of egg-laying it becomes 

 more than two feet in length and nearly half an inch in width, thus 

 increasing the volume about fifty times. This remarkable growth 

 occurs annually. Gadow remarks also that the testes of the house- 

 sparrow increase from the size of a mustard-seed to that of a small 

 cherry, and in so doing temporarily displace the usual arrangement 

 of the viscera in the body-cavity. 2 



A very large number of birds seasonally migrate, and this habit, 

 as in the case of the migratory fish already referred to, is closely 

 associated with the function of breeding. 3 Jenner 4 stated long ago 

 that migration was invariably associated with an increase in size of 

 the ovaries and testes, and that when these begin to shrink, after 

 discharging their functions, the birds take their departure. Thus 

 the ovaries of the cuckoo are stated to be almost atrophied in July. 

 It would seem quite possible that the annual development of the 

 sexual organs is the immediate stimulus which, in the individual, 

 fixes the time for the spring migration, for it is known that in birds 

 passing northward the ovaries and testes are well developed. (But 

 cf. fishes, p. 17). Thus wading birds, such as the sanderling shot by 

 Dr. Eagle Clarke at Spurn Head, in May, were found by him to have 

 their sexual organs in a very advanced state of growth. These birds 

 were probably on their way to Greenland or Siberia. 



Schafer 5 has suggested that the migratory impulse is determined 

 by the relation of daylight to darkness, having been brought into 



1 Simpson, "An Investigation into the Effects of Seasonal Changes upon 

 Body Temperature," Proc. Roy. Soc. Edin., vol. xxxii., 1912. That the periodicity 

 is not simply due to temperature is shown by the Cereopsis goose of Australia 

 which, when brought to this country, lays its eggs in the autumn and hatches 

 them in the winter, or at the same time as the Australian summer. Cf. black 

 swans, see p. 24. 



2 Gadow, Article on "Reproductive Organs," in Newton's Dictionary of 

 Birds, London, 1893-96. Disselhorst also ("Gewichts- und Volumszunahmen 

 der mannlichen Keimdrusen," Anat. Anz., vol. xxxii., 1908) has called attention 

 to the enormous increase in size and weight of the testicles and ovaries in 

 many birds (and also in some Mammals) in the breeding season. Thus, in 

 Fringilla, the testicles may increase three-hundredfold. 



3 For much of the information given here regarding migration, I am in- 

 debted to Dr. Eagle Clarke. 



4 Jenner, "Some Observations on the Migration of Birds," Phil. Trans., 

 Part I., 1824. See also John Hunter, Animal (Economy, London, 1786. 



6 Schafer, "On the Incidence of Daylight as a Determining Factor in 

 Bird Migration," Nature, November 7, 1907. 



