The European Shag (Phalacrocorax aristotelis) breeds in a wide range of nest-sites depending on the locality, such as crevices under fallen rocks, open ground caves and open ledges on craggy cliffs. In this paper the habitat selection of shags breeding in cavities on the coastal slopes of Islas Cíes (Galicia, Northwest Iberian Peninsula) are examined. Shags selected sites with more lateral and overhead cover, with better drainage and with average visibility. In addition, sites where breeding was successful differed from unsuccessful sites. Nest-site characteristics especially affected the hatching success. In this colony, shags showed adaptive responses to site-quality variability. Thus, nest-site quality declined with density and with seasonal occupancy. Shag colonies seem to follow an ideal despotic distribution, where some individuals monopolize high quality sites and prevented other individuals from settling in the good sites. Further studies are required to assess the proximal mechanisms used for nest-site selection in this species.