AVIAN MIGRATION AND REPRODUCTION 695 



remained to be determined. However, before experiments were under- 

 taken to determine this, another experiment was performed to test the 

 results of the above experiments. 



If there is a physiological preparation on short days for a subse- 

 quent response to long days, then it should be possible to induce a 

 response several times within one year, even though juncos normally 

 show only one period of gonadal activity and two periods of fat 

 deposition annually. Production of repeated cycles was attempted by 

 exposing birds to alternating periods of short days (9L-15D) and 

 long days (20L-4D) (Wolf son, 1954). The lengths of the periods 

 varied, but in a total of 369 days, juncos were exposed to four periods 

 of short days and five periods of long days, including the initial 

 natural long days of April. Five periods of gonadal activity (includ- 

 ing the initial active state when the experiment began), five periods 

 of fat deposition, and two molts occurred within the 369 days. These 

 are indicated in Fig. 2. Gonadal growth, fat deposition, and increase 

 in body weight were correlated with long days. Gonadal regression, 

 loss of fat deposits and decrease in body weight were correlated with 

 short days. These correlations became less distinct as the experiment 

 progressed. Figure 3 shows the marked changes in body weight and 

 fat deposition during the first periods of short and long days. Prior 

 to this study. Rowan (1929), Miyazaki (1934), Damste (1947), 

 and Burger (1947) had induced more than one period of gonadal 

 activity in a year in different species of birds. In Fig. 2, duration of 

 response is indicated by horizontal bars except in a few cases where 

 they indicate only occurrence on the date of observation. Fat response 

 indicates presence of medium or heavy fat deposits. Molt response is 

 comparable to the annual molt. S. refers to occurrence of sperm, 

 which were obtained by applying pressure to the seminal vesicles 

 (Wolfson, 1952c). 



The results of this experiment confirmed the conclusion of the 

 previous series on the refractory period that short days in the fall 

 regulate a reaction that enables the bird to respond to subsequent 

 photoperiodic treatment. Without this "preparatory period" the bird 

 does not respond. Although the bird is "refractory" to long days in 

 the fall, it is undergoing a response which is regulated by day length 

 (short days) and which is necessary for a subsequent response. Hence, 



