Variation in altitudinal migration winter segregation and site tenacity in two subspecies of dark eyed juncos junco hyemalis in the southern appalachians usa
Rabenold, K.N.; Rabenold, P.P.
Auk 102(4): 805-819
ISSN/ISBN: 0004-8038 Accession: 006892415
We studied altitudinal migration of Dark-eyed Juncos (Junco hyemalis) over 5 yr in the Great Smoky Mountains to test hypothesis concerning ecological determinants of winter distribution and distance traveled in migration. We individually marked 1,832 juncos belonging to two subspecies that occur together in winter in the foothills: Carolina Juncos (J. h. carolinensis) that breed in high-elevation spruce-fir forests locally and Northern Juncos (J. h. hyemalis) that are latitudinal migrants. Carolina Juncos spend the winter at higher elevations than Northern Juncos. Above 600 m elevation in the drainage that formed the study area, juncos were 76% Carolinas in winter on average. Carolina Juncos show winter assortment by sex across altitudes that parallels latitudinal assortment found among Northern Juncos by Ketterson and Nolan (1976, 1983). Most Carolinas wintering above 600 m elevation and within 20 km of ridgetop breeding habitats were males (77%). Farther downslope most juncos were Northerns (83%) and most Carolinas were females (80%). Some males are resident year-round in the breeding habitat, while others migrate through the entire altitudinal range. Differential altitudinal migration by the sexes of Carolina Juncos and altitudinal segregation of the two races in winter were variable between years; smaller and competitively subordinate classes of juncos were better represented at higher elevations in a milder winter. This variation and the patterns of survival and ranging revealed by recapture data were consistent with the hypothesis that social dominance in competition for food significantly affects winter distribution. Migration and choice of wintering ground in this system are flexible responses that are probably malleable by pressures created by behavioral interactions with other birds in competition for winter food and breeding territories as well as by physiological constraints. Althrough three hypotheses developed in studies of latitudinal migration predict the basic patterns of winter assortment by subspecies, sex, and size, the balance of selective forces is likely different for these altitudinal migrants.