AVIAN MIGRATION AND REPRODUCTION 699 



or less. Whether the duration of the preparatory phase is related to 

 the length of the short day has not been determined, but a few relevant 

 data suggest that there may be some relation between the length of 

 the dark period and the rate at which the preparatory phase proceeds. 



After the preparatory phase has been completed, the progressive 

 phase begins. In nature, it probably begins automatically, but the rate 

 at which it proceeds is governed by day length. The occurrence of the 

 spring premigratory physiological state was induced in juncos by 

 mean's of long days in about 40 days. This was about 80 days ahead 

 of its naturaroccurrence. With a short day (9L-15D) the same state 

 appeared in about 160 days, which was 40 days later than its occur- 

 rence in nature. The maximum rate of response occurred with a day 

 length as short as 15.5 hr; the minimum rate occurred with a day 

 length of 9 hr, but shorter day lengths were not tried. Nine hours was 

 used as the minimum, because this is about the shortest day to which 

 juncos are exposed in nature. 



It is important to emphasize here that the progressive phase pro- 

 ceeded when there was no increase in day length. When the birds were 

 exposed to 9-hr photoperiods beginning December 4, they had already 

 undergone the preparatory phase under natural day lengths. On De- 

 cember 4 their day length was reduced from approximately 10 hr 

 under natural conditions (including civil twilight) to 9 hr, and it re- 

 mained constant at 9 hr thereafter. Yet, these birds achieved the 

 spring premigratory physiological state, on the basis of their fat deposi- 

 tion and body weight. Their reproductive organs were, however, some- 

 what below the normal. Would such birds actually migrate? During 

 1957 over 200 birds were held on 9-hr days from December to June 

 and then released. A preliminary analysis of the data shows that about 

 60% undertook a migration. 



In view of our earlier knowledge which showed the stimulatory 

 effect of increasing day lengths during the winter, it must be empha- 

 sized that gradually increasing day lengths (or an increase in day 

 length) are not necessary to induce spring migration. The role of day 

 length, once the birds are ready to respond, is the regulation of the 

 rate at which the response proceeds. The progressive phase proceeds 

 spontaneously without increases in day length and when the days are 

 as short as the shortest days in December and remain short. This is a 



