Unlike the breeding season, during which breeding birds are presumably more easily separated from mere migrants or summer visitors, the differences between wintering birds and winter birds are very subtle in this season. This led us to ask ourselves whether this Atlas should deal with birds wintering in Catalonia (ecological approach) or with winter visitor birds (temporal approach). To produce a wintering bird atlas it is necessary to have an accurate knowledge of every species winter phenology. However, this creates a conceptual difficulty: what is the dividing line between a late migrant and a wintering bird? How should be treated birds that breed in the winter time, such as the Lammergeier, the Eagle Owl, etc.? According to this, it is seemingly much easier, and still applied, to produce an Atlas of those bird species present during the time that in a broad sense we come to call winter period (the period falling between the large post and prenuptial migratory fluxes, roughly from November 15th until February 15th). The Atlas title reflects this conceptual framework and it leans towards the winter rather than the wintering bird concept.