The Hawaiian Hawk or Ilo (Buteo solitarius) is a medium-sized, broad-winged buteonine en- demic to the Hawaiian Islands. Although this hawk was believed to be threatened with extinction and included on the Endangered Species List in 1967, little was known about its population status or breeding ecology at the time of the listing decision. Information about the distribution, abundance, and biology of the Hawaiian Hawk is anecdotal (Banko 1980), with the exception of systematic surveys by Scott et al. (1986) and Hall et al. (1997). Fossils of Hawaiian Hawks have been found on Hawaii, Molokai, and Kauai (Olson and James 1997), and there have been eight documented observations of the species on the islands of Kauai, Oahu, and Maui since 1778 (Banko 1980, Olson 1990). Since ornithologists have studied the Hawaiian Islands, Hawaiian Hawks have nested only on the island of Hawaii from sea level to 2,600 m (Banko 1980, Scott et al. 1986). The Hawaiian Hawk is among the most sexually dimorphic of the world's buteonine raptors (Paton et al. 1994). Objectives of the present study were to quantify nesting habitat, breeding behavior and chronology, nest success, diet, and other factors affect- ing reproduction.