Hawaiian Hawk

Buteo solitarius

Peter LaTourrette
  • ACCIPITRIDAE
  • Hawks, Osprey, Harriers, Eagles, Kites
  • 2,150

Although the Hawaiian Archipelago was once home to an eagle, a harrier, and a hawk, only the Hawaiian Hawk, or 'Io, managed to survive the human colonization of the Hawaiian Islands. Today, the Hawaiian Hawk is listed as a federally endangered species, threatened by illegal shooting, harassment of nesting birds, and degradation of native forest nesting habitat.

Identification
Hawaiian Hawk is the only species of hawk found in Hawaii. It has two different color morphs--one light and one dark--which are roughly equal in abundance. Dark-morph adults are dark brown overall (often appearing black in the field), with a gray tail lightly barred with brown. Light-morph adults are also brown above and have a gray tail, but they have pale underparts with brown streaks on the breast. Immature light-morph birds are a striking golden buff on the head and breast with dark upperparts.

Distribution and Population Trends
This species is endemic to the island of Hawai'i, although vagrants occasionally wander to Maui, O'ahu, and Kaua'i.

Population estimates for Hawaiian Hawk range from 1,600 to 2,700 individuals. It is difficult to assess population trends for this species due to a lack of information on historical numbers.

Ecology
Hawaiian Hawk is found in a wide variety of habitats, from exotic forest and pastureland in the lowlands to native forest as high as 8,900 feet in elevation. Most successful nesting, however, is confined to higher elevation native forest with 'ohi'a trees. The nest is large and bulky, consisting mainly of twigs which are piled without organization until the bowl of a nest is formed. Historically, clutches of one to three eggs were reported, but most nests found during recent studies of the hawk's breeding biology have contained just one egg. Incubation lasts at least 38 days, and young birds leave the nest about 60 days after hatching. Breeding pairs are extremely aggressive in defense of the nest.

This species has a varied diet, which includes insects, introduced mammals, and native and non-native species of birds. Unfortunately, it preys upon young Hawaiian Crows, adding another conservation concern to that already highly-endangered species. Hawaiian Hawks hunt from a perch or, more rarely, in flight. This species is most commonly seen soaring on thermals above forest or agricultural land either singly or in pairs. Hawaiian Hawk shows the most dramatic size difference between sexes of any species of Buteo hawk, with females being larger than males.

Threats
It is believed that disturbance of nesting birds and the illegal shooting of Hawaiian Hawks might be the most important threats facing this species, but it is difficult to accurately determine the level of shooting and trapping. The degradation of native forest habitat is another significant threat to this species. Introduced pigs and other ungulates have had a devastating impact on native Hawaiian plant species, directly reducing their numbers while also facilitating the spread of exotic plants that then out-compete remaining native plants. Hawaiian Hawks show a strong preference for nesting in native 'ohi'a trees, but this tree species is almost completely absent on Hawai'i below an elevation of 2,000 feet, due to competition from introduced plants.

Conservation
Hawaiian Hawk was listed as Endangered under the federal Endangered Species Preservation Act in 1967. In 1984, the U.S. Fish and Wildlife Service published the Hawaiian Hawk Recovery Plan, which included the following recommendations: (1) monitor population status; (2) maintain suitable habitat for feeding and nesting, including conservation of remaining native forest habitat (primarily 'ohi'a and koa forests); (3) enforce prohibition of taking; and (4) evaluate potential impacts of changes of pesticide use. In 1993, the Fish and Wildlife Service proposed changing the status of the hawk to Threatened, based on breeding studies of the species. However, based on the uncertainty that remains regarding Hawaiian Hawk's population trend, this proposal to downlist the species was later withdrawn, and additional population and breeding surveys were recommended. Population surveys are currently being conducted to establish population trends, and research studies are examining the breeding success of the species in different habitat types. These surveys and studies should provide valuable information in determining whether or not Hawaiian Hawk should be recategorized as a Threatened species.

What Can You Do?

The Endangered Species Act has helped protect the Hawaiian Hawk and made it possible to learn critical information about its biology. Audubon continues to work to ensure that this vital legislation is being used to protect our publicly-owned wildlife resources. Check out http://policy.audubon.org/ to learn of the latest news about the Endangered Species Act and how you can help. To learn more about other species protected under this legislation, visit: http://endangered.fws.gov/

U.S. National Wildlife Refuges provide essential habitat for the Hawaiian Hawk, and a great number of other species throughout the U.S. and its territories. Unfortunately, the refuge system is often under-funded during the U.S. government's budgeting process.  To support U.S. National Wildlife Refuges, consider purchasing a duck stamp which helps to fund wetland habitats in refuges: http://www.fws.gov/duckstamps/

References
BirdLife International (2000) Threatened Birds of the World. Barcelona and Cambridge, UK: Lynx Edicions and BirdLife International.

BirdLife International (2006) Species factsheet: Buteo solitarius, Hawaiian Hawk http://www.birdlife.org/datazone/species/index.html?action=SpcHTMDetails.asp&sid=3512&m=0

Clarkson, K.E. and L. P. Laniawe. 2000. Hawaiian Hawk (Buteo solitarius). In The Birds of North America, No. 523 (A. Poole and F. Gill, eds.). The Birds of North America, Inc., Philadelphia, PA.

Pratt, H.D., P. L. Bruner, and D. G. Berrett. The Birds of Hawaii and the Tropical Pacific. Princeton: Princeton University Press, 1987.