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Hawaiian stilt (ae`o)

The Hawaiian stilt (Himantopus mexicanus knudseni) prefers to nest on freshly exposed mudflats with low growing vegetation. Nesting may occur in fresh or brackish water and in either natural or manmade ponds. With the exception of Lanai, Ka-ho‘olawe and possibly Hawai‘i, the stilt historically inhabited all the major Hawaiian Islands. They currently occur on all the main islands except Ka-ho‘olawe. The largest population occurs on O‘ahu, primarily on the north and windward coast at Kahuku Point on James Campbell National Wildlife Refuge, Kahuku Point oyster ponds, Amorient aquaculture ponds, Roland Pond and at Nuupia Ponds in Kaneohe. Smaller populations exist at Pearl Harbor and along the leeward coast. On Kaua‘i, the subspecies is found in large river valleys such as Hanalei, Wailua, and Lumahai, on the Mana Plain, and at reservoirs and sugarcane effluent ponds in Lihue and Waimea. On Maui, the largest groups occur on the Kanaha and Kealia coastal wetlands. Other populations occur in reservoirs and aquaculture areas. On Molokai, birds occur in south coast wetlands and playa lakes. The species colonized Lanai in 1989 where it occurs in Lanai City's wastewater treatment ponds. On the island of Hawaii, the largest populations occur on the Kona coast from Kawaihai Harbor south to Kailua. It also occurs in the Makalawena and Aimakapa Ponds, Cyanotech Ponds and Kona wastewater treatment ponds. Smaller populations occur along the Hamakua Coast and in the Kohala River valleys of Waipio, Waimanu, and Pololu. Anchialine ponds along the Kona coast provide prime feeding sites.

While no historic population estimates exist, the species was formerly quite common. It declined to 200 birds by 1941 due to habitat loss, predation, and human hunting, but climbed to about 1,000 birds by 1949, apparently in response to release from hunting pressure [1]. The 1941 estimate has been questioned by some due to the much higher 1949 estimate, but modeling indicates the species is capable of explosive growth under good conditions [3].

Winter and summer surveys have been conducted on various islands since 1956. A review of trends from 1956 to 1989 [4] showed that: Summer population estimates were more variable on an island basis and were considered less reliable than winter counts. However, the statewide summer population count (excluding Kaua'i/Ni'ihau which were not surveyed until 1975) declined significantly from 1968 to 1979, and then increased significantly from 1980 to 1989. The winter counts showed that the Maui population increased significantly from 1956 to 1989, being relatively stable from 1956 to 1971 then increasing from 1972 to 1989. The Moloka'i population increased significantly from 1968 to 1989. The Hawaii population declined significantly from 1968 to 1976, and then increased significantly from 1977 to 1989. The O'ahu population declined significantly from 1956 to 1968, and then increased significantly from 1969 to 1989. The Kaua'i/Ni'ihau population showed no trend between 1975 and 1989 (but the study had little power to detect a trend due to sample size). The statewide population (excluding the Kaua'i/Ni'ihau population) increased significantly between 1968 and 1989.

The U.S. Fish and Wildlife Service corrected errors in past biannual survey data, concluding that the statewide population increased significantly between 1976 and 2003 [2]. The total population is currently between 1,500 to 1,800 birds. The 1978 final, 1985 revised, and 2005 draft revised recovery plans call for a stable or increasing population of at least 2,000 for downlisting and delisting [2].

[1] Robinson, J. A., J. M. Reed, J. P. Skorupa, and L. W. Oring. 1999. Black-necked Stilt (Himantopus meicanus). In The Birds of North America, No. 449 (A. Poole and F. Gill, eds.). The Birds of North America, Inc., Philadelphia, PA
[2] USFWS. 2005. Draft Revised Recovery Plan for Hawaiian Waterbirds, Second Draft of Second Revision. U.S. Fish and Wildlife Service, Portland, Oregon. 155 pp.
[3] Reed, J.M, C.E. Elphic, and and L.W. Oring. 1998. Life history and viability analysis of the endangered Hawaiian stilt. Biological Conservation 84:35-45
[4] Reed, M.J. and L.W. Oring. 1993. Long-term population trend of the endangered Ae'o (Hawaiian stilt, Himantopus mexicanus knudseni). Transactions of the Western Section of the Wildlife Society 29:1993(54-60).
[5] Bureau of Sport Fisheries and Hawai`i Department of Land and Natural Resources. 1970. Hawaii’s Endangered Waterbirds. Bureau Sport Fisheries and Wildlife, Department of Interior, and Hawai`i Division of Fish and Game, Department of Land and Natural Resources. Portland, OR.
[6] Shallenberger, R.J. 1977. An ornithological survey of Hawaiian wetlands. U. S. Army Corps of Engineers Contract DACW 84-77-C-0036, Honolulu, HI. 406 pp.

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