We studied Thick-billed Parrot (Rhynchopsitta pachyrhyncha) nest-site density and social nesting behavior from 1998 to 2001 in Madera, Chihuahua, Mexico. The species formed high-density nesting clusters; 45 nesting attempts (30%) involved nesting pairs sharing nest trees, with a maximum of three nesting pairs per tree. The majority of nest trees were live or dead quaking aspens (Populus tremuloides). Clusters contained a mean of 11.5 breeding pairs (5 nests/ha). The highly social nesting behavior of Thick-billed Parrots may have important implications for management and conservation of their breeding habitat.