770 REPRODUCTION AND MIGRATION IN BIRDS 



Moore and Mehrhof (1946) compared the egg production of hens 

 subjected to periodic increments in light ration with that of hens un- 

 der continuous light. The periodic increases consisted of 2-hr addi- 

 tions, at 14-day intervals, to the initial natural day (October in 

 Florida). Periodic increments effected a greater initial response in 

 egg production than did all night lighting, but this differential was 

 not maintained and the overall performance of hens under continuous 

 light was slightly though not significantly superior. It would be inter- 

 esting to know if smaller increments than those used by Moore and 

 Mehrhof would prove stimulatory over a longer time. 



Byerly and Moore (1941) compared the effect of 14 hr light, 12 

 hr darkness (26-hr day) with that of 14 hr light, 10 hr darkness (24- 

 hr day). Rate of production was considerably greater for hens on the 

 26-hr day. The authors believed their results to "show conclusively 

 that it was possible to lengthen the clutch* by synchronizing dark 

 and light periods with the hen's natural ovulation cycle," thus suggest- 

 ing something akin to an endogenous rhythm. As far as is known, this 

 interesting suggestion has not been investigated further. 



(b) Intermittent lighting. Roberts and Carver (1941) reported 

 that 3 hr of intermittent light daily ( 1 light, 5 dark — 1 light, 4 dark 

 — 1 light, 12 dark) yielded production above that of 10 hr continuous 

 light. Dobie et al. (1946) compared the effect of 10 hr continuous 

 light with a base day of 8 hr to which a supplemental period of 2 hr 

 was interposed at differing times within the 14 hr (total) darkness. 

 The effect of the interposed 2 hr light was increasingly effective as the 

 maximal period of darkness was decreased. In a similar experiment 

 based on a total day length of 13 hr, no significant differences were 

 observed. Dobie et al. concluded that "if thirteen hours of light are 

 furnished, a period of eleven hours of darkness is not too long for 

 best production." 



Wilson and Abplanalp (1956) compared the effects of intermittent 

 light and unbroken light at total durations believed to be suboptimal 

 for egg production. Intermittent light was given at 4-hr intervals, 90, 

 60, 45, 15, 5, and 1 min during each interval. Generally, intermittent 



* The term "clutch" refers here to the number of eggs laid on consecutive 

 days. 



