Prior to the arrival of humans, the Hawaiian Archipelago was home to a harrier, an eagle, at least 4 species of owls, and a small, broad-winged hawk (the Hawaiian Hawk or 'Io). Excepting the Short-eared Owl (Asio flammeus), which colonized the islands in the post-Polynesian era ( Olson and James 1991 ), the 'Io is the only raptor in the Hawaiian Archipelago that survived into the historical period. Since the Hawaiian islands lacked terrestrial mammals when the ancestor to the 'Io colonized Hawai'i, it likely relied predominantly on aerial pursuit for most prey captures. The 'Io is one of the most sexually dimorphic members of Buteo . Selection for increased maneuverability to capture small avian prey has likely led to this high degree of dimorphism ( Paton et al. 1994 ).
Although the fossil record indicates that 'Io were former residents of the islands of O'ahu, Moloka'i, Kaua'i, and Hawai'i ( Olson and James 1997 ), they are now found only on the island of Hawai'i. Why the 'Io has survived there is an enigma; its historical range does not appear to have changed ( Banko 1980b , J. Klavitter pers. comm.). It was federally listed as Endangered in March, 1967.
'Io are considered 'aumakua-family or personal gods of the Hawaiian culture. The 'Io is also a symbol of Hawaiian royalty because of its lofty flight ( Pukui and Elbert 1986 ).
Titian Peale ( Peale 1848 ) first described the species from a single specimen collected near Kealakekua. Banko ( Banko 1980b ) summarized historical accounts and distributional records of the species. Griffin ( Griffin 1985 , Griffin et al. 1998 ) conducted a 3-yr study of life history with emphasis on breeding biology. Hall et al. ( Hall et al. 1997b ) conducted an islandwide survey for the 'Io and assessed its abundance and distribution. The U.S. Fish and Wildlife Service (USFWS) is currently continuing population surveys and examining demography and nesting success by vegetation type to evaluate criteria for down-listing the species.