A Mediterranean population of the Eurasian nuthatch Sitta europaea using nestboxes was studied during three breeding seasons (2009-2011). The study area was a managed holm oak forest where suitable natural cavities for breeding were accordingly scarce. A large proportion of breeding territories was significantly associated with nestboxes located in the periphery of the patch and with large-holed nestboxes. The relationship between habitat heterogeneity (predicted by canopy cover of holm oaks and tree numbers) and breeding performance (egg-laying date, clutch size, hatching success, fledging success and breeding success) was tested. Egg-laying date was independent of predictors of habitat heterogeneity but was related to breeding success: earlier nesters achieved better breeding success than those breeding later. Clutch size and breeding success showed a negative relationship with canopy cover and tree numbers, respectively: both reproductive parameters were higher in the apparently suboptimal territories than in the optimal ones. These results are interpreted as an effect of the severe competition for cavities between all species breeding in the nestboxes and the reproductive output of nuthatch is density-dependent. The high overall breeding success achieved by nuthatches suggests that holm oak forest must be considered a high quality habitat for this species.