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Timing of Autumn Migration and Its Relation to Winter Distribution in Dark‐Eyed Juncos

Nolan, Val, Jr., Ketterson, Ellen D.
Ecology 1990 v.71 no.4 pp. 1267-1278
Junco, adults, autumn, birds, breeding, females, males, migratory behavior, prediction, wintering grounds, Indiana
Migratory Dark—eyed Juncos in eastern North America migrate southward in autumn and tend to segregate in the winter range according to sex and age. North to south, the most abundant classes are young males, adults males, young females, and adult females. Because adults tend to dominate young at winter feeding sites, this distribution appears at first to conflict with the view that dominance interactions are responsible for differential avian migrations and that dominant individuals remain nearest the breeding range. However, if young juncos establish winter residency at earlier dates than adults, a prior residence effect might make them dominant. This hypothesis requires that young arrive at wintering sites earlier than adults and that residents of all classes arrive earlier than their transient counterparts. We tested these predictions by comparing first—capture dates of juncos at Bloomington, Indiana, which is north of the latitudianal midline of the junco's winter range. During 13 autumns, individuals were classed as local residents or an transients and were sexed and aged. Contrary to predictions were not caught earlier than transients. Further, adults were caught earlier, not later, than young among transiets and probably also among residents. Thus, settlement of the winter range does not proceed from north to south, and dominance established through prior residence cannot account for the concentration of 1st—yr males in the northern part of the winter range. The fact that some sex—age classes tend to winter south of others predicts that at a northern capture site the classes that migrate farthest should be commoner among transients than among residents. This expectation was fulfilled. In addition, the median capture dates of the sex—age classes were arranged approximately according to the north—to—south order of their distribution, indicating that classes with the farthest to travel passed through (transients) or settled (residents) earliest in autumn. We conclude that comparison of autumn migration schedules of transiet and resident passerine birds at a single location can yield considerable information about the dynamics of settlement of the entire winter range, including possible information about differences in destination of subsets of transients.