Black Harrier Circus maurus is a rare southern African endemic that may have lost over 50% of its core breeding habitat in the last century as a result of extensive land transformation by agriculture, invasive alien vegetation and urbanization in the Fynbos biome. We partially surveyed both the western (Swartland) and southern (Overberg) coastal plains of south-western South Africa, over 3 years (2000–2002) for breeding Black Harriers, and found a distinctly polarized distribution. Nests were concentrated either along the coastal strip or inland in montane habitats, and generally absent from heavily cultivated and transformed inland plains areas. Limited evidence (direct observations, prey remains) suggests that harriers forage in cereal croplands but generally do not breed in these modified environments. We recorded breeding success at nests in coastal (Dune Thicket) and montane (Mountain Fynbos) habitats. Harriers bred successfully along the coast and nests were aggregated in loose colonies around wetlands. Harriers in montane environments bred poorly, took a wide range of prey, and were subject to high levels of nest predation. We propose that Black Harriers have been displaced from lowland Renosterveld and Fynbos habitats (characterized by better foraging and nesting opportunities), primarily by the advent and spread of cereal agriculture. The conservation and future research implications of this hypothesis are discussed.