732 



REPRODUCTION AND MIGRATION IN BIRDS 



cially induced, during which no photoperiodic response can be induced 

 regardless of the length of the daily photoperiod. This phenomenon 

 was noted by Riley (1936) in Passer domesticus, and by Schild- 

 macher (1938a) in Phoenicurus phoenicuriis. The approximate times 

 of the termination of the refractory period for some passerine species 

 under natural conditions are recorded in Table II. Figure 5 gives a 

 more detailed illustration for a single species, Zonotrichia leucophrys 

 gambelii. There appears to be little or no information on the natural 

 duration of the refractory period in nonpasserine species. It appears 



Table II. Natural Termination of Refractory Period of Passerine Species of 

 Northern Hemisphere (Modified from Farner, 1954) 



that the multiple cycles obtained in a single year by Miyazaki (1934) 

 with Zosterops japonica, by Wolfson (1954) with Zonotrichia leuco- 

 phrys and Junco hyemalis, by Damste (1947) with Chloris chloris, 

 and by Burger (1947) with Sturnus vulgaris are, in part, attributable 

 to regression resulting from reduction of the daily photoperiod before 

 the development of refractoriness. Interpretations of this sort must 

 be developed with caution, since, at least in some species, the dura- 

 tion of the refractory period is inversely related to the duration of 

 the daily photoperiod (Burger, 1949; Wolfson, 1952b, 1954; Vaugien, 

 1954b). Apparently also in some species refractoriness will continue 



