Abstract : Understanding species habitat requirements is a central issue in biodiversity conservation and management and it is even more important with the actual global change. In this context, we study nest site selection of a semi-colonial ground-nesting species breeding in farmlands, the Montagu's Harrier Circus pygargus. We use different spatial and temporal scales to understand how environmental variations affect patterns of vegetation height selection at the nest, their underlying processes and individual fitness. The selection of an optimal vegetation height at the nest depends on two main risks whose relative importance changes in a breeding season: predation and overheating risks of eggs and chicks. Montagu's Harrier selects tall vegetation at nest (i.e. 80 cm). However, variations of the three components of habitat (vegetation phenology and growth, prey abundance and presence of conspecifics) affect to varying degrees selection patterns of vegetation height and breeding parameters. The importance of the semi-colonial character of the species is further emphasized: conspecific attraction and among-pair competition strongly influence selection patterns at different spatial and temporal scales. Conspecific attraction acts at large scale while among-pair competition appears at finer scale. In the light of these new elements, we conducted a discussion about the future of Montagu's harrier in intensive agricultural areas in the context of global change.