The spatial and social organization of a winter communal roost of Rooks (Corvus frugilegus L.) was examined. The behaviour of rooks in the roost was found to reduce the rate of loss of heat energy for all or some of the population. The saving of energy experienced by young birds by roosting in sheltered positions was important because of the smaller energy reserves available. When the weather became exceptionally severe, dominant individuals, in seeking sheltered positions, forced less dominant birds from more sheltered to far less sheltered positions, thereby increasing their energy loss. The effects of this selectively-acting social hierarchy during the night is exacerbated on the feeding grounds during the day when some rooks starve especially those low in the hierarchy. The daytime feeding situation will also increase the lower critical metabolic temperature of the young rooks by lowering the nutritional plane. Other evidence is presented to show that in winter, mortality of young rooks is higher than that of adult rooks.

It is argued that the change in the spatial organization in the roost induced by weather and determined by a selectively-actingsocial hierarchy operates to reduce the rate of energy loss of the majority of the rooks while the remainder suffer an increasing energy deficit.