AVIAN MIGRATION AND REPRODUCTION 707 



able that there is a preparatory phase in these birds that is regulated 

 by day length rather than being a self-regulated spontaneously ex- 

 pressed rhythm. Unlike the junco, these migrants would not experi- 

 ence days shorter than about 12 to 13 hr, or nights longer than about 

 11 to 12 hr. Is this a short enough day or a long enough night to 

 complete a preparatory phase? Since some juncos could "prepare" on 

 a constant 12L-12D schedule, it is probable that these migrants could 

 also prepare under the photoperiodic conditions they would experi- 

 ence while migrating from their North Temperate breeding grounds 

 to their wintering grounds from September to November. Since only 

 six weeks of treatment with 12L-12D in July and August was enough 

 to complete the preparatory phase in the junco, it is highly probable 

 that equatorial and transequatorial migrants would be exposed to a 

 sufficient number of 12- to 13-hr days during the period of fall migra- 

 tion to complete their preparatory phase. An alternative explanation 

 is that the effective day lengths during the preparatory phase are 

 longer than those in the junco. Species differences in the relation be- 

 tween day length and the preparatory and progressive phases are to 

 be expected. It is interesting to note that in nature juncos winter where 

 they will experience dark periods longer than 12 hr; hence, spontane- 

 ous initiation of the progressive phase is assured. The regulation of 

 the preparatory phase and of the initiation of the progressive phase is 

 the critical problem in the regulation of spring migration in equatorial 

 and transequatorial migrants. 



Once the preparatory phase is over, it is highly probable that the 

 long days of the equatorial region and the Southern Hemisphere, 

 whether they were relatively constant, increasing, or decreasing, would 

 regulate the rate at which the progressive phase proceeded. 



Engels (p. 759) has presented a few data on the growth of the 

 testes in relation to photoperiod in an excellent transequatorial 

 migrant, the bobolink (DoUchonyx oryzivorus), but the data are 

 probably applicable to the premigratory physiological state. His data, 

 although not conclusive, suggest the existence of a preparatory phase 

 which requires short days for its completion and which is prevented 

 by long days. As in the junco, day lengths of 12 hr were short enough 

 for the occurrence of the preparatory phase, but 10-hr days were more 

 effective during treatment for eight weeks. Photoperiods of 1 4 hr per 



