Does dominance determine how far dark-eyed juncos, Junco hyemalis, migrate into their winter range?

The behavioural dominance hypothesis suggests that differential migration among individuals of a species of bird is due solely to social interactions that force subordinate individuals (often a class, e.g. female or young birds) to migrate farther into the winter range than dominant individuals (often a class, e.g. male or old birds). Here, this hypothesis was tested with two experiments. In the first experiment, the prediction was tested that dominance acts within a sex-age class and influences migration distance. The outcomes of interactions within dyads of dark-eyed juncos, Junco hyemalis, were observed. Each dyad consisted of a junco caught in winter in Michigan matched with another of the same sex-age class caught in Indiana, which is situated farther south and therefore farther into the winter range of this species. Michigan birds were dominant in only half of the experimental dyads (21 of 41 dyads), which is inconsistent with the prediction of the dominance model. In the second experiment the prediction was tested that members of a sex-age class that migrates farther into the winter range should be subordinate to members of a different class that migrates a shorter distance. Young males that wintered in Michigan were pitted against old males that wintered in Indiana. In 19 of 25 dyads, the more southern-wintering old males were dominant, which also is counter to the prediction of the dominance hypothesis. These results indicate, at the very least, that in migratory J. h. hyemalis, dominance does not play as important a role in determining latitude of winter residence as has been suggested. Migration is one form of behaviour by which social dominance status. According to the hypothe- animals that breed at temperate latitudes may sis, if resources on the breeding grounds are escape the rigours of winter. Differential migration inadequate for an extended period such as a winter in temperate-breeding birds occurs when indi- season, then dominant birds will travel only as far viduals of the same species, but different sex-age into the winter range as is necessary to find classes, migrate varying distances into the winter- sufficient food, and subordinate birds will move ing grounds. Differential migration of sex-age farther in order to avoid competition with the classes has been found in a wide variety of avian dominant birds. The result predicted by the species (reviewed by Ketterson & Nolan 1976, hypothesis is a linear relationship between poten- 1983, 1985; Myers 1981; Gauthreaux 1982). In tial dominance status and distance migrated into general, females and young overwinter farther into the winter range. No studies to date have directly the wintering range than males and adults. tested this prediction of the dominance hypothesis. Various hypotheses have been advanced to explain such intraspecific variation in avian migra- tion (for review see Gauthreaux 1982; Ketterson & Nolan 1983). The behavioural dominance hypo- thesis (Gauthreaux 1978, 1982) states that in social competition for resources, the sex-age classes experience unequal success because they differ in