This paper examines some consequences of changes in the membership and location of winter flocks of Dark-eyed Juncos (Junco hyemalis). Among captives, intragroup social relationships seldom were affected by separation from or subsequent reunion with another portion of the home flock, by amalgamation with or separation from a foreign flock, or by changes in flock location. Foreign flock members usually were subordinate to, and preferentially attacked by, residents. Juncos rejoining their home flocks after a 1-week separation retained their previous social positions but engaged temporarily in a disproportionate amount of agonistic behavior with former flockmates. Some proximate and ultimate implications of winter flock stability in juncos are discussed in the light of these and other findings. Social and ecological costs that may be incurred by juncos attempting to change their flock membership or home range during the winter should select for behavior that reduces the likelihood of such changes, including geographic differentiation in the migratory habits of the sexes.