The Role of Migration and Winter Mortality in the Life History of a Temperate-Zone Migrant, the Dark-Eyed Junco, as Determined from Demographic Analyses of Winter Populations

ABSTRCT. --This study of migratory Dark-eyed Juncos (Junco h. hyemalis) investigates dif- ferences in winter population structure and dynamics associated with north-south variation in wintering site and considers the relationships between these winter differences and population dynamics at other seasons. Banded juncos were captured and recaptured in several winters at four latitudes (42o-33.5o), and results were integrated with long-term United States Fish and Wildlife Service records of recoveries throughout the winter range. The data suggest: (1) Populations wintering in the north experience higher winter mortality than populations wintering in the south. (2) Sex and age do not affect the prob- ability of overwinter survival; therefore, dominance may not affect that probability. (3) Some individuals do and others do not return to the same wintering site year after year. Individ- uals that do not return tend to be those that have spent the first winter of life in the north, and these shift southward. (4) Annual mortality of northern and southern winter populations probably is the same. This conclusion is based in part on a comparison of north-south annual return rates and on logic arising out of the geographical distribution of the sexes during winter. In juncos, because males tend to winter north of females, greater annual mortality of northern winter populations would be expected to skew the primary and sec- ondary sex ratios in favor of males. No evidence of such skews has been detected. (5) If annual mortality is the same, it is likely that southern populations suffer heavier mortality during their longer migrations and that these migration losses offset the advantage described in (1), above. In north temperate-zone breeding species that migrate southward into an extensive winter range, distance traveled probably is correlated closely and positively with both mortality rate during migration and survival rate during winter. If these rates offset each other, annual survivorship among winter migratory populations (and among sedentary and migratory populations of partially migrant species) can be equal. Therefore, it is unnecessary to pos- tulate for different winter populations either unequal reproductive success (von Haartman's hypothesis) or unequal year-to-year survival rates that balance out over the long term (Lack's hypothesis). Migration can cause breeding populations to be redistributed in various ways in winter; e.g. they may mix randomly, or northern breeders may leapfrog southern breeders. Each variation in migration pattern would produce winter populations with predictable charac- teristics. We consider the extent to which winter junco populations have these character- istics. Received 26 January 1981, accepted 30 September 1981. THE Dark-eyed Junco (Junco h. hyemalis), an abundant ground-feeding emberizid, migrates between a breeding range lying largely in the boreal forests of Canada and, in eastern North America, a winter range reaching from south- ern Ontario and the northern boundary of the United States almost to the Gulf of Mexico (Bent 1968). Because the winter range is so ex- tensive, northern-wintering juncos spend that critical season (Lack 1954, Fretwell 1972) in cold and often snow-covered regions, whereas oth-