In northwest Massachusetts, black-capped chickadees shifted foraging groups repeatedly each day during the winter. As a result, there was no clear distinction between flock members and floaters, but instead a continuous gradation from birds that formed relatively stable associations to those that shifted groups more frequently. Group territories were absent and individual territories were at most poorly defended, if present at all. This picture differs from other recently studied populations, where flock membership is stable and flock territories are clear. Comparison with other populations indicates that winter social structure in the black-capped chickadee may be sensitive to both winter food levels and the proportion of year-round residents. This indicates that cost-benefit analyses of winter social organization should consider factors acting throughout the year as well as the residence status of the wintering birds. The data suggest that flock size, population density and home range size are sensitive to winter food on the study area, whereas territoriality and flock cohesion may respond more clearly to the local availability of breeding resources or the proportion of year-round residents.