AN ENDOGENOUS BASIS FOR DIFFERENTIAL MIGRATION IN THE DARK-EYED JUNCO

Male and female Dark-eyed Juncos, Junco hyemalis hyemalis, partially s egregate along a latitudinal gradient in North America during winter. The sex ratio favors males in the north and females in the south. The sexes differ, not only in their midwinter distribution, but also in th e timing of fall migration, with females passing through stopover site s earlier than males. To investigate the basis for these differences i n migration patterns, several measures of migratory behavior, includin g the date of onset and the duration of migratory activity (Zugunruhe) , were compared in captive adult male and female juncos. When held und er the same photoperiod and given free access to food, females initiat ed fall Zugunruhe twelve days earlier than males and continued to be a ctive after the males stopped. This resulted in females showing, on av erage, 21 more days of migratory activity. There was no difference bet ween the sexes in the proportion of nights during the migration period spent exhibiting migratory activity or in the average amount of activ ity per bird for the season. Females briefly showed higher migratory a ctivity in early September. Both sexes exhibited a significant increas e in body fat at the onset of migratory activity but there was no sign ificant difference between the two groups at any time during the sampl ing period. Although differences in social status, physiological const raints, and/or importance of early arrival have been proposed as both past and current factors influencing differential migration in many sp ecies, these data illustrate sufficient endogenous sex-related differe nces that can result in differential migration in Dark-eyed Juncos.