Habitat Use of Hawaiian Waterbirds in Kawainui Marsh

Abstract

In Kawainui Marsh, located on the windward side of O‘ahu, eleven man-made ponds were developed by the U.S. Army Corps of Engineers and the Hawai‘i Department of Forestry and Wildlife (DOFAW) to supplement nesting and foraging habitat for endangered, native Hawaiian waterbirds ( ae'o - Hawaiian stilt (Himantopus mexicanus knudseni), ʻalae keokeo - Hawaiian coot (Fulica alai), and 'alae 'ula - Hawaiian gallinule (Gallinula galeata sandvicensis). Management of hydrological conditions is thus a priority for DOFAW, as robust water circulation is critical to provide optimal wetland habitat for native waterbirds, as well as to prevent outbreaks of avian botulism caused by natural toxins produced in wetland soils. A better understanding of Hawaiian waterbird use of managed wetland habitat in relation to water quality is needed to implement effective management strategies. The objectives of this project are to: (1) compare water quality parameters among ponds within the Kawainui Marsh pond system; (2) determine the relationship between water depth and water quality; (3) evaluate habitat use of Hawaiian waterbirds in relation to water depth and water quality. Weekly surveys were conducted in the wetland pond system from May 2018 through March 2019, between the hours of 8:00 AM and 11:00 AM, to measure water quality parameters (temperature, dissolved oxygen [DO], pH, salinity, turbidity, and oxidation-reduction potential [ORP]) using a YSI ProDSS sonde. A census of the number of Hawaiian waterbirds and their behaviors was recorded in the pond system concurrently during water quality surveys. Data was divided into wet and dry seasons, which correlated with precipitation. Two-way analysis of variance (ANOVA) tests were used to compare water quality parameters and presence of waterbirds between season and among ponds, as well as test for an interaction between season and pond. Regression models were used to determine the relationships between water depth and (1) water quality parameters, and (2) waterbird behaviors. Hawaiian coots were observed most frequently during the wet season, when water depths were greatest, and primarily utilized the North Ponds, which had the greatest number of ponds with water throughout both wet and dry seasons. Hawaiian coots showed a preference for ponds with deeper water, particularly when foraging; however, no observations of nesting Hawaiian coots, and very few observations of resting Hawaiian coots, were recorded during this study period. Maintaining water depths above one foot may increase the number of foraging Hawaiian coots during the dry season. Hawaiian stilts were frequently observed in both the wet and dry seasons; however, Hawaiian stilts were observed nesting only during the dry season, which is likely because the dry season coincides with the Hawaiian stilt nesting season. Hawaiian stilts utilized the North and South Ponds for foraging, but primarily utilized the North Ponds for nesting, resting, and preening. The greatest number of nesting stilts were observed in pond 11, which may be due to the number of small islands that provided preferred nesting habitat. Hawaiian stilts do not seem to be limited by water depth, particularly when foraging; however, additional data on nest-site characteristics is needed to inform management decisions for Hawaiian stilt habitat. Hawaiian gallinules were not frequently observed during this study period, but when observed, gallinules primarily utilized the North Ponds for foraging in both wet and dry seasons. One Hawaiian gallinule was observed nesting in pond 4 during the dry season, which could be due to the availability of dense vegetation throughout that pond. Hawaiian gallinules may prefer ponds with deeper water, as they were most frequently found foraging in ponds with water depths of one to two feet. Overall, our results suggest maintaining water depths in Kawainui Marsh is most important for providing foraging habitat for Hawaiian coots and nesting habitat for Hawaiian stilts; however, more data collection is needed to determine specific water depths.