Maintenance of viable populations requires detailed knowledge of a species' area demands and of the mechanisms influencing them. From 1992 to 1996, I examined, in northern Switzerland, the relations between habitat factors and home range size of 33 radio-tracked middle spotted woodpeckers (Dendrocopos medius), an endangered species confined to mature oak forests. Stepwise backward multiple regression revealed that home range size decreased from winter to late spring; furthermore, range size was inversely correlated to the densities of large oaks (greater-or-equal, slanted36 cm dbh) and of potential cavity trees. Neither intra- nor interspecific competition seemed to influence home range size. Jackknife and cross-validation procedures indicated good predictive capability of the habitat model. Therefore, both the availability of large oaks, being related to potential food abundance, and of potential cavity trees, being related to reproduction, appear to be used as proximate cues to determine home range size. Management decisions concerning the endangered middle spotted woodpecker have focused on the conservation and promotion of large oaks; in the future they should consider the supply of trees suited for cavity excavation as well.