Communal roosting is known to decrease predation risk and thermoregulatory costs, and to increase foraging efficiency or both. While the advantages associated with roost site selection compared to nearby areas have been studied, the factors ruling the selection of roosts within a roosting site remain largely unknown. We investigated what factors affect the preference for roosts within a winter roosting site of Brambling Fringilla montifringilla. Type of vegetation, microclimate, topography and sources of disturbance were considered as possible variables affecting site selection and thus the density of birds. The density of droppings estimated at each of 300 sampling plots was used as an index of density of birds within the roosting site. We modeled density of birds using Generalized Additive Models with different explanatory variable combinations. Results show that factors shaping site selection are similar to those affecting habitat use at other scales, with Brambling resting in higher densities in warmer areas. This can be explained by the protection from winds and radiative heat loss during night, in accordance with results of previous studies at broader scales. In addition, we found a negative impact by human activities, Brambling avoided areas close to roads and forest tracks, possibly as a consequence of local traffic but also the of the affluence of people to watch the birds. Results suggest that site selection is best explained by the interaction between variables rather than by individual variables.