Why does MagpiesPica pica roost communally rather than solitarily in winter? Most roosts were situated in deciduous trees near water, and they held relatively small numbers of birds. All Magpies roosted communally in winter. Strong winds reduced the height of roost perches, and roosting in leeward conditions increased. Less time was spent at the roost before and following roosting in winter when compared with the autumn and the spring situations. Magpies from high quality territories arrived late in the evening and left early in the morning. Foraging activity increased at the roost as day-length decreased, whereas flying remained constant. Sexual and aggressive behaviour were common in spring. Juveniles were sub-ordinate at the roost. Communal gathering was common at the roost. Several activities occurred frequently either at the pre-roost or at the roost, but aggressive and sexual behaviour had a spatial distribution as expected from the number of birdhours spent in the two sites. Roosting ceased following initiation of nesting activity, but early nesters attended the roost longer than late nesters following start of nest building. Start of breeding was not synchronized within roosts. Predation was rare at the roost and elsewhere in the study area. Roosting seems not to be due to information centre effects, predator avoidance, favourable micro-climate, lack of roost sites, or synchronization of breeding activities. Magpies may monitor the quality of their neighbours including their mating status at the roost. Communal roosting of corvid species seems to be related primarily to the distribution and the abundance of food, species with an unpredictable food supply roosting communally.