Effect of Long Days on Molt and Autumn Migratory State of Site-faithful Dark-eyed Juncos Held at Their Winter Sites

-Dark-eyed Juncos (Bunco hyemalis) caught at their regular winter sites and then held outdoors fail to become restless or to fatten in the following autumn. Two alternative explanations for this failure to manifest the autumn physiological migratory state are (1) that the birds’ perception of familiar characteristics of the migratory destination suppresses the autumnal state, or (2) that spring and summer daylengths in the winter range are too short to permit birds held there to complete the annual physiological cycle. We caught juncos in winter on their perennial winter home ranges, held them there during spring and summer, and then monitored their restlessness and fattening next autumn. We divided these winter-caught birds into three treatment groups. The first two were alike in being exposed indoors to long days simulating the photoregime of spring and summer on the breeding range. They differed in that we placed one of the two groups outdoors in view of the natural environment for ten days in late summer, just before monitoring began, whereas during those ten days the other group remained indoors and on the local photoregime until monitoring. The third group was held outdoors throughout spring and summer on the natural daylengths of their winter site. We compared these three site-faithful groups with each other and with juncos that we caught on the breeding range in late summer, displaced to the winter range, and monitored alongside the other groups. The summer-caught juncos fattened and became restless. The site-faithful group that remained indoors until monitoring exhibited a slight tendency to do so. The site-faithfuls that were placed outdoors after long days indoors and the site-faithfuls that spent spring-summer outdoors neither fattened nor became restless. Site-faithful groups exposed to artificially long days molted earlier than birds recently caught on the breeding range and also earlier than the site-faithfids continuously held outdoors. Recently caught birds probably delayed molt because, although they had experienced long summer days, they had also reproduced. Our results indicate that the previously reported suppression of the autumnal migratory state in juncos held outdoors at the destination of the autumn migration is not attributable to the shorter daylengths the birds experience there during spring and summer. They are less helpful on the question of whether the suppression results from recognition of the winter site. Received 8 Dec. 1988, accepted 16 Nov. 1989. Internal timing mechanisms are important for first-time migrants of some species. Such mechanisms apparently maintain the young bird in autumn migratory condition for a period sufficient to allow it to travel to the winter range of its species or population (e.g., Gwinner 1969, 1986; Berthold 1973, 1975; Berthold and Quemer 198 1; Ketterson and Nolan 1986). Much less is known of the regulation of subsequent migrations, I Dept. of Biology, Indiana Univ., Bloomington, Indiana 47405.