700 REPRODUCTION AND MIGRATION IN BIRDS 



major change from the conclusions of the earlier experiments in which 

 migration was induced out of season by subjecting birds to artificial 

 increases in day length (Rowan, 1929; Wolfson, 1942, 1945). 



After the birds are in the premigratory physiological state, the 

 length of time they remain in this state is a function of day length. 

 Long days dissipate the state more quickly than short days, and under 

 short days ( 12L-12D and 9L-15D) the birds may not lose their fat. A 

 change from long days to short days will also dissipate the state 

 rapidly (Fig. 3). 



In nature, the spring premigratory physiological state ends when 

 the birds arrive on their breeding grounds, or shortly afterward. After 

 the breeding season, the gonads regress, the birds molt, and subse- 

 quently, there is a physiological change, which precedes the onset of 

 fall migration. Nothing is known about the factors which regulate 

 this state. When the fall migration gets underway in September and 

 October the day lengths have reached a value which is effective for 

 the beginning of the preparatory phase of the next spring migration. 

 And thus a new cycle begins. 



Role of Light and Darkness in the Migratory Cycle 



The available data suggest that in the preparatory phase, the effec- 

 tive part of the short days is the dark period. It seems likely that there 

 is a dark-dependent response which requires a daily uninterrupted 

 dark period of at least 12 hr duration. This response cannot occur 

 when a greater total amount of darkness is given per 24 hr in smaller 

 doses, for example 4L-8D-4L-8D. Or, if the dark-dependent reaction 

 goes on during short dark periods, it cannot summate to give an 

 efTective daily response, or a response after the lapse of many days. 

 The absence of an inhibitory reaction produced by long days is appar- 

 ently not a factor, since with short periods of light, the same small 

 total amount of light per day, and the same ratio of light to darkness 

 the preparatory phase did not occur (for example, 4L-8D-4L-8D 

 was negative, whereas 8L-16D was positive; 6L-6D-6L-6D was nega- 

 tive, whereas 12L-12D was positive). However, more studies are 

 needed to establish this point, because of the possibility of carry-over 

 effects of the light periods. The responses induced by each daily 

 effective period of darkness summate. Eventually, the bird completes 



