726 



REPRODUCTION AND MIGRATION IN BIRDS 



very rapidly a substance which is essential for the photoperiodic 

 response and that during the dark period this essential substance 

 decays at a rate much slower than its rate of formation so that the 

 stimulation of testicular development persists temporarily after the 

 end of the photoperiod. We (Farner, Mewaldt, and Irving, 1953a,b) 

 have referred to this period of continued effect during the dark as 

 the "carry-over period." Our data require the assumption that k is 

 effectively a positive function of the concentration of the hypothetical 



0.080 



0.060 - 



K' 



0.040 - 



0.020 - 



0.000 



8 12 16 



total light per day in hours 



20 



24 



Fig. 3. A comparison of the rates of testicular development for single 

 daily photoperiods and equally spaced 50-min photoperiods in first-year 

 Zonotrichia leiicophrys gambelii. See equation (1) for definition of rate 

 constant, k. Light intensity is supramaximal. 



substance only up to a concentration somewhat lower than equilibrium 

 concentration during the photoperiod. The data suggest that a period 

 of the order of a minute is required for the process to go from dark 

 concentration to equilibrium concentration of the photoperiod, and 

 that the time required for the substance to decay back to minimum 

 dark concentration is of the order of a few hours (Farner and Wilson, 

 1957a). This hypothesis, which has value in considering the nature 

 of the response mechanism and in the design of experiments, is con- 



