The density and behavior of the Mistle Thrush in Niepołomice Forest (southern Poland) and adjacent open areas were studied during winter (December—February) in 1996/97, 1998/99 and 2000/01. Mistle Thrush densities differed significantly from winter to winter, and the abundance of thrushes decreased as the season progressed. Bird density and mistletoe clump density were correlated positively. Birds held territories or congregated in flocks. The latter were sighted in the forest only during winter 1996/97, when the largest density of birds was noted. Flock size decreased progressively during that winter, but at the same time, the number of territorial birds remained stable. This suggests that by the end of the winter 1996/97 some birds from the flocks had begun to hold territories. Flocks were also seen in open areas, and displayed a preference for foraging on pastureland. Each individual territory in the forest consisted of several clumps of mistletoe on a few adjacent trees, which were defended against both conspecifics and other species such as Pyrrhula pyrrhula, Turdus merula and Dendrocopos major. The aggressive encounter rate was correlated positively with bird density but negatively with the progress of winter (the latter was correlated negatively with the berry supply). Surprisingly, it was not correlated with mistletoe clump density or temperature. During abundant berry years, the density of birds may have been so large that defending the fruit against numerous neighbors would have been energetically less profitable than communal foraging.