Abstract.  Unprecedented cavity nesting among falconids: a Chimango Caracaras (Milvago chimango) population using Borruwing Parrots (Cyanoliseus patagonus) nests in Patagonia Argentina. The Chimango Caracara (Milvago chimango) is the most abundant member of raptor assemblages from Paraguay and Bolivia to the southern tip of Argentina. In Argentinian Patagonia we studied nesting and other biological aspects of a Chimango Caracaras population that exploits cavities built by marine psittacines, in a unique ornithological assemblage: 12 km of sea cliffs with 37,000 active nests of Burrowing Parrots (Cyanoliseus patagonus). During 56 days we assessed a total of 18 active Chimango Caracaras nests (11 in 2014 and 7 in 2015). All nests contained plant material inside and had some structure to facilitate landing and perching. Most were located in the upper third of the cliff and had a single entrance mouth. The average distance to the nearest nest in the same breeding season was 108.9 m ± SD 90.90 m. We document in detail for the first time the construction of Chimango Caracaras nests inside cavities. Our results document a new breeding substrate for this species and a nesting behaviour unique among falconids, which could be interpreted as evidence of adaptability and plasticity of Chimango Caracaras.