The Hawaiian Stilt (Himantopus mexicanus knudseni), or Ae‘o, is an endangered subspecies of the Black-necked Stilt (Himantopus mexicanus) which inhabits wetlands throughout the Hawaiian Islands. Hawaiian Stilts frequently move among wetland habitats in search of food, but are limited by habitat characteristics, such as water depth. Research on Hawaiian Stilt foraging requirements is limited, and information provided by the state of Hawai‘i and US Fish and Wildlife Service differs. Additionally, the Hawaiian Stilt is threatened by sea level rise, which has increased flooding events by raising the water table, particularly in coastal communities. Due to this threat, it is important to determine possible impacts of increased water depths on Hawaiian Stilt foraging success.
In this study we aimed to:
Determine the relationship between water depths and (1a) number of foraging stilts, (1b) foraging activity and (1c) foraging success;
(2) compare morphological characteristics of the Black-necked Stilts to those of the Hawaiian Stilt.
Mitchell, C, C Ogura, DW Meadows, A Kane, L Strommer, S Fretz, D, Leonard, and A McClung. October 2005. Hawaii’s Comprehensive Wildlife Conservation Strategy. Department of Land and Natural Resources. Honolulu, Hawai‘i. 722 pp.
Rotzoll, K., & Fletcher, C. H. 2012. Assessment of groundwater inundation as a consequence of sea-level rise. Nature Climate Change, 3(5), 477–481.
U.S. Fish and Wildlife Service. 2011. Recovery Plan for Hawaiian Waterbirds, Second Revision. U.S. Fish and Wildlife Service, Portland, Oregon. xx + 233 pp
1)Eight observational surveys were conducted over thirty days between the months of February and March, 2018 in Kawanui and Hamakua Marshes on the windward side of O‘ahu.
2)Observations were randomized using a grid with eight blocks (A-H) and four cells within each block. Surveys began in the block with the least number of stilts to minimize disturbance and were continued in a clockwise direction. If no stilts were present in the cell, this was noted and the observer moved to the next cell.
3)A water depth category (1, 2, 3) was assigned to each cell in relation to positions along the stilt’s leg. Category “1” represents from zero to the top of the foot (~0-2 cm); category “2” represents the top of the foot to the ankle (~2-15 cm); category “3” represents the ankle to the hip (~15-22 cm).
4)Total number of strikes and number of successful strikes were each recorded during a continuous observation period lasting 2 minutes per individual, or until the individual flew away, whichever was less.
5)A Chi-squared test was used to determine the relationship between presence of foraging stilts and water depth. An ANOVA was used to determine the relationship between water depth and (1) successes per strikes per minute, and (2) strikes per minute. A t-test was used to compare the mean tarsus length of the Hawaiian Stilt to the Black-necked Stilt.
The presence of foraging stilts was significantly dependent on water depths that were below the ankle (<15 cm; Hamakua: c22 = 44.45; P < 0.001; Kawainui Marsh: c21 = 14.69; P<0.001). In both marshes stilts were observed in water depths ranging from the ankle to the top of the foot (~ 2-15 cm). Foraging activity (number of strikes) was not significantly different between water depths in Hamakua (F2 = 2.39; P = 0.11 ), and not significantly different between water depths in Kawainui (F1 = 1.44; P = 0.24 ). Foraging success rate (successes per strikes per minute) was not significantly different between water depths in Hamakua (F2 = 0.41; P = 0.67 ), and not significantly different in Kawainui (F1=0.78; P = 0.39). Mean tarsus length of the Black-necked Stilt (118.5 mm; Hayman, Marchant, and Prater, 1986) was not significantly greater than the mean tarsus length of the Hawaiian Stilt (120.3 mm +/-6.64) (t20 = 1.21; P=0.24).
Our study was designed to determine suitable water depths for foraging by Hawaiian Stilts, particularly in light of concerns regarding the loss of suitable habitat due to sea level rise.
Although foraging activity and foraging success did not significantly differ among water depth categories in Hamakua Marsh or Kawainui Marsh, the number of foraging stilts significantly differed among water depths in both wetlands. In fact, stilts were only observed foraging in water depths below the ankle. (<15 cm). Mean tarsus length of the Black-necked Stilt did not significantly differ from the Hawaiian Stilt, suggesting that recommended water depths for Black-necked Stilts are likely suitable for the Hawaiian Stilt.
Our results suggest that foraging by Hawaiian Stilts may be impacted by sea level rise, as they are likely to lose the existing foraging habitat at the suitable depths of under 15 cm. Based on this study, the availability of these shallow water depths should be maximized for stilt population stability.
We thank the Hawai‘i State Department of Land and Natural Resources. We thank Hawai‘i Wildlife Ecology Lab members for their assistance in data collection and photography. This project is made possible by funding from the UROP Program at the University of Hawai‘i, Mānoa.