722 



REPRODUCTION AND MIGRATION IN BIRDS 



0.10 



0.05 



doily photoperlod in hours 



Fig. 2. The rate of testicular development as a function of the daily 

 photoperiod in Zonotrichia leucophrys gambelii. See equation ( 1 ) for 

 definition of rate constant, k. Open circles represent samples of first-year 

 birds; closed circles represent adjusted means for samples of adults. Light 

 intensity is supramaximal. (From Farner and Wilson, 1957b.) 



k approaching zero are almost negligible. However, the nature of the 

 lower tail of the curve in Fig. 2 does have obvious theoretical interest 

 and should be subjected to further investigation. The linear portion 

 of the curve in Fig. 2 may be approximated by 



"■Ist yr. 



0.009(/? - 9.1) 



(2) 



It would be of substantial interest to examine other species with re- 

 spect to the functional relationships between Wt and t as well as be- 

 tween p and k. The data of Winn (1950) for Junco hyemalis, 

 although somewhat more variable than those for Zonotrichia leuco- 

 phrys gambelii, suggest a conformance with equation (1). The same 

 is possibly true for those of Burger ( 1948) for Stumiis vulgaris. Our 

 own analysis of the data for Passer domesticus from Bartholomew 

 (1949), Vaugien (1952a,b), and Farner and Wilson (unpublished) 



