An Endogenous Basis for Differential Migration in the Dark-Eyed Junco

Abstract

Male and female Dark-eyed Juncos, Junco hyemalis hyemalis, partially segregate 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 the timing of fall migration, with females passing through stopover sites earlier than males. To investigate the basis for these differences in migration patterns, several measures of migratory behavior, including the date of onset and the duration of migratory activity (Zugunruhe), were compared in captive adult male and female juncos. When held under the same photoperiod and given free access to food, females initiated fall Zugunruhe twelve days earlier than males and continued to be active after the males stopped. This resulted in females showing, on average, 21 more days of migratory activity. There was no difference between the sexes in the proportion of nights during the migration period spent exhibiting migratory activity or in the average amount of activity per bird for the season. Females briefly showed higher migratory activity in early September. Both sexes exhibited a significant increase in body fat at the onset of migratory activity but there was no significant difference between the two groups at any time during the sampling period. Although differences in social status, physiological constraints, and/or importance of early arrival have been proposed as both past and current factors influencing differential migration in many species, these data illustrate sufficient endogenous sex-related differences that can result in differential migration in Dark-eyed Juncos.