ARTICLES ON HAWAIIAN BIRDS AND BIRDWATCHING AND OTHER PACIFIC WILDLIFE



The Hawaiian Stilt - Distribution and Population Status



The Hawaiian Stilt is a large, slender shorebird which is black above and white below and with a white forehead and variable white spot above the eye. It has long pink legs and a long dark bill. Male birds have jet black upperparts whilst females have browner feathering on their backs. Females also have lower pitched voices. The Hawaiian sub-species differs from the mainland form in having more extensive black around the forehead and around the sides of the neck (Coleman 1981). They also differ in the length of their bill, tarsus and tail with the bill and tarsus length being the longest of all the Stilt complex (Prater et al. 1986). Juveniles are mottled brown, grey and white at first but quickly moult into a more adult plumage where they can be distinguished by leg colour and less black on the cheeks.

The Hawaiian Stilt was first described in 1888 by L. Stejneger from specimens supplied by V. Knudsen, hence the scientific name. The Hawaiian Stilt is currently regarded as a subspecies of the Black-necked Stilt (
Himantopus mexicanus mexicanus), but evidence suggests that it may be better considered a full species.

Historic Range and Population Status

The Hawaiian Stilt is currently found on Kaua'i, O'ahu, Maui, Moloka'i, Hawai'i and when wetlands are available Ni'ihau. Prior to 1900 the species was reported to be common to abundant on all the main islands except Hawai'i by early naturalists, although it was also recorded as declining during this period. The species was found utilising shallow ponds and tidal flats. In 1946 and 1947 the first statewide count was initiated and about 1,000 individuals were recorded. Current population levels appear to be highly dependent on rainfall patterns as they are for Hawaiian Coot. Stilts will readily move to newly opened or disturbed sites where invertebrate numbers are high and easily obtained. The Statewide Stilt population was estimated by Engilis & Pratt (1993) to consist of between 1200 and 1600 individuals, Kaua'i, O'ahu and Maui supporting 92% of the population.

O'ahu population

On O'ahu the species is recorded as being stable and numbers between 1956 and 1981 show a slight increase with average counts of between 275 and 536. The species on this island have been noted (Banko 1988) as increasingly using managed wetlands, whereas the use of unmanaged wetlands is decreasing.
Several areas which formerly held numbers of Stilts into the 1940's and 1950's have been destroyed and must have had an impact on distribution of Stilts on O'ahu. At present O'ahu supports the largest population in the Islands and occur mainly on the North and windward coasts, with between 40 and 60 percent of the State total occurring on this island.

Moloka'i population

Moloka'i has only a small amount of suitable waterbird habitat and therefore not surprisingly the numbers of Stilt are relatively low, though numbers are stable. Banko (1988) noted that there appeared to be a distinctive pre-breeding exodus and post-breeding influx of birds, presumably suggesting that the island cannot support large numbers of birds during the breeding season due to territory conflict and perhaps insufficient food. Current population estimates for the island place the population at about 125 individuals (Ducks Unlimited surveys 1993 to 1995).

Maui Population

Maui has a sizeable Stilt population but is limited by the lack of extensive suitable habitat, most birds are present in the Kanaha Pond - Kealia area, and due to the very shallow water here the nesting success of the area is also very high compared to O'ahu and Kaua'i. Numbers of Stilts in the 1956 - 1981 period on average ranged between 189 and 487 individuals and between 200 and 500 individuals since 1983.

Hawai'i population

No Hawaiian Stilts were known from Hawai'i prior to 1961 and by 1981 had only a small population of Stilts with an average of between 19 and 28 birds. Most birds are restricted to the Kona coast with other sites only being visited occasionally. The present population is estimated at 130 birds along the Kona coast (Ducks Unlimited surveys 1996 to 1997).

Lana'i Population

A pair of stilts was first observed on Lana'i in 1989 at the wastewater treatment ponds (DOFAW, unpubl. Obs 1988 - 1996) and the population now numbers more than thirty birds (State department of Land and Natural Resources waterbird survey records 1989 to 1996). Limited habitat availability at present will mean that the population on this island will not increase very much more.

Kaua'i Population

Banko (1988) reported that use of Kaua'i wetlands had increased dramatically since 1956, when regular record keeping began. He also reported that summer counts were higher than winter counts (almost double) and explained this as post - breeding dispersal from Ni'ihau of adults with young.
Banko also noted that a change in habitat use was occurring with an increase in numbers using the dry West leeward side, such as Kekaha and fewer birds using the North windward shore areas such as Hanalei. Having said this however he also stated that Stilts for the majority of their time inhabit agricultural areas such as settling basins, taro fields and wet pastures, but tended not to use reservoirs; this would appear to show some change then in the habitat quality, not quantity of areas such as Hanalei and the North Shore where amounts of this habitat were increasing.
Banko also stated that few Stilts nested on Kaua'i, less than ten pairs annually, and that survival of young was low. This again would imply that the habitats available were not providing appropriate nesting and more importantly feeding areas, especially for chicks and young birds. However, Banko's suggestion that only ten pairs breed on Kaua'i falls far short of the number nesting in the period 1998 - 2001. At Hanalei NWR on the North Shore up to 50 pairs bred during the period, although survival of chicks was low, although poor nest and chick searching may have made the numbers counted even lower than they really were. Other sites which had breeding pairs (by area) were: North Shore: 5 -10 pairs; East Side: 3 - 5 pairs; South and Southeast side: 10 - 15 pairs and west side: 7 - 15 pairs. Although these figures are estimates, it shows that approximately 95 pairs (maximum) nest on the island. Winter counts reveal numbers in excess of 350 birds during the period 1998 - 2001.

Once again information on specific habitat and feeding requirements is very scarce for the Hawaiian Stilt. Population, distribution and nesting behaviour have been covered well (Coleman 1981, Reed and Oring 1991 and Reed et al. 1998) and go some of the way towards an understanding of the species particular requirements, they do not however provide the information needed to create or manage habitat that will support individuals once they are attracted to a site and begin to breed and nest.

Studies of Black - necked Stilts (Himantopus mexicanus) have also mostly been applied to population and nesting behaviour, although Cullen (1994) investigated Stilt use of different depth water ponds in Puerto Rico, and this provides a good insight into tropical and sub-tropical use of habitat by the species. Results show however that there is a vast difference between tropical continental and it's near islands and remote island species requirements.

Aquatic habitat and aquatic food sources for endangered waterbirds at Hanalei NWR by Broshears and Parrish (1980), Bird use of Hanalei Taro fields relative to agricultural cycle by Byrd, Moriarty and Bautista (1979) and Strategies for managing endangered waterbirds on Hawaiian National Wildlife Refuges by Chang (1990) provide the most complete and useful information on feeding requirements of native Hawaiian wetland birds. The two former studies provide details of food preference and feeding styles and the latter includes a comparative study of a site pre and post manipulation, however the latter research assumed that increased nesting and rise in numbers of birds present was indicative of increased production and improvement of the habitat all round, no feeding data was collected though.

The Hawaiian Stilt was listed on October 13th 1970 (35 Federal Register 1607)

Occurrence of Hawaiian Stilt in the Hawaiian Islands.
NI'IHAU KAUA'I O'AHU MOLOKA'I LANA'I MAUI HAWAI'I
YES* YES YES YES YES YES YES
* Species present usually only during times of adequate rainfall on this island

Hawaiian Stilt chick at Hanalei NWR, Kaua'i. June 2000.

References

BANKO, W.E. 1988. Historical synthesis of recent endemic Hawaiian birds. Part I. Population histories - species accounts, freshwater birds: Hawaiian Stilt, Ae'o. Cooperative National Park Resources Studies Unit, University of Hawaii at Manoa, CPSU/UH Avian History Report No.12.

BROSHEARS, R.E and J.D. PARRISH. 1980. Aquatic habitat and aquatic food sources for Endangered waterbirds at Hanalei NWR. Hawaiicooperative Fishery Reasearch Unit Technical Publication 80-3. Final Report .

BYRD, G.V., D. MORIARTY and N. BAUTISTA. 1979. Bird use of Hanalei Taro fields relative to agricultural cycle. Unpublished progress Report. Project Hal 1 - 78.

CHANG, P.R. 1990. Strategies for managing endangered waterbirds in Hawaiian National Wildlife Refuges. M.Sc. Thesis, University of Massachusetts, Cambridge, Massachusetts.

COLEMAN, R.H. 1981. The reproductive biology of the Hawaiian subspecies of the Black-necked Stilt, Himantopus mexicanus knudseni. Ph.D. diss. Pennsylvania State University. University Park, Pennsylvania

CULLEN, S.A. 1994. Black-necked Stilt foraging site selection and behavior in Puerto Rico. Wilson Bulletin, 106 (3). 1994

DOFAW, 1988 to 1996. Waterbird survey records. Unpublished data on file at Division of Forestry and Wildlife, Honolulu, Hawaii

DUCKS UNLIMITED.1993 to 1997. Survey data for Hawaiian waterbirds. Unpublished data on file at Ducks Unlimited, Sacremento, California

ENGILIS, Jr. A. and T.K. PRATT 1993. Status and population trends of Hawaii's native Waterbirds,1977-1987. Wilson Bulletin, vol. 105 (1) pp 142-158

PRATER, A,; J. MARCHANT and P. HAYMAN. 1986. Shorebirds: An identification guide. Houghton Mifflin, Mass

REED, J.M. and L.W. ORING. 1991. Long-term population trends of the endangered Ae'o (Hawaiian Stilt, Himantopus mexicanus knudseni). Proceedings of The Wildlife Society, Western Section. 1991

REED, J.M., M.D. SILBERNAGLE, K.EVANS, A.ENGILIS and L.W. ORING. 1998. Subadult movements of the endangered Hawaiian Stilt (Himantopus mexicanus knudseni). The Auk.


Christian Melgar. Worthing, West Sussex, UK. 2002.


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©Birding Hawaii 2002