768 REPRODUCTION AND MIGRATION IN BIRDS 



a dozen eggs in late spring or early summer. He notes further that 

 most fancy breeds in which there has been little or no selection for egg 

 production behave similarly. The effect of light in these domesticated 

 birds is thus to time the appearance of a strictly limited breeding 

 season, as in wild birds generally (Marshall, 1936, 1942, 1956; 

 Rowan, 1938). 



It is common knowledge that in breeds and strains of domestic 

 fowl developed for egg production, the restricted breeding season (in 

 its rigorous sense) has been eliminated. Hammond (1954) summar- 

 izes the changed role of season succinctly. ". . . the fowl has been so 

 modified by selection that a season can now be defined in only a sta- 

 tistical sense: the effect of light is observed not on extent of season 

 but on relative rates of production." Rate of production is measured 

 as the number of eggs laid over a given number of days, expressed in 

 percent. An egg a day over any number of days thus signifies a rate of 

 100%. 



Natural Photoperiods. The effects of seasonal photoperiodicities 

 are well defined in most fowl. In a now classical paper, Whetham 

 (1933) described the correlation between day length and rate of Qgg 

 production over a wide range of latitudes. Onset of seasonally increas- 

 ing production generally preceded onset of longer days; this "incom- 

 plete correlation" was thought to arise out of low relative rates of 

 change in light ration preceding onset of longer days. Whetham noted 

 also that the effect of increasing day length was greater in low- than 

 in high-producing hens. 



In early maturing, high-producing breeds or strains, pullets hatched 

 in March or April may begin to lay during September or October, 

 i.e., during the season of decreasing — even rapidly decreasing — day 

 length. Such birds are not "short-day" animals; day length, though 

 decreasing, is adequate to assure sexual maturity. If the birds are 

 maintained under natural light, production declines or ceases after a 

 time and subsequently exhibits the usual seasonal increase associated 

 with lengthening days. 



Artificial Photoperiods. The value of supplemental light in alter- 

 ing Qgg production has long been known. Baker and Ranson (1932) 

 mention the application of light by the Spaniards over a century be- 

 fore 1932 for this purpose. Speaking presumably of this country, 



