Wetlands in Hawai'i are highly productive ecosystems distributed along the coastal plains of the major islands, providing ecosystem functions such as natural water filtration of nutrients and chemicals, flood protection, recharge groundwater and as shoreline protection by reducing erosion risk (Barbier et al. 1997; Finlayson et al. 1999; Levin et al. 2001; Blackwell and Pilgrim 2011; de la Hera et al. 2011). These wetlands are created by accumulated rainfall, streamflow, and upwellings of groundwater and support a variety of endemic wildlife. Hawaiian wetlands are typically located along level coastal plains with abundant freshwater and are therefore often in prime areas for human development. O’ahu, the state's most developed island, has suffered a loss of 75% of coastal wetlands (van Rees and Reed 2014). The ephemeral wetlands on O’ahu are managed by Federal and State agencies as core waterbird habitats (USFWS 2011).
Hawai’i had 26 species of endemic waterbirds, 21 have gone extinct and all remaining species are endangered (Olson & James 1994; USFWS 2011). Currently, all remaining Hawaiian waterbirds persist in a series of geographically isolated patches of habitat. A major factor in the longer-term conservation of these endangered species is the connectivity among populations. Of these birds, the Hawaiian stilt is found on all the major Hawaiian Islands year-round (Banko 1988; Reed & Oring 1993; Underwood et al. 2013). The island of O’ahu supports the largest population of Hawaiian stilt (HDOFAW 1980-2008; USFWS 2011; van Rees & Reed 2014). Hawaiian stilts have demonstrated to be a highly mobile species making them difficult to track across urban environment (Reed et al. 1994, 1998b; Hickey et al. 2007). Little is known about how habitat and landscape-level factors affect their movements and use of habitat. To understand the movement behavior and connectivity of Oahu's Hawaiian stilt. We used the novel approach of outfitting Hawaiian stilts with GPS solar satellite tracking devices to estimate and quantify the use of different habitats of these birds and understand the impact of Oahu's modern landscape on their habitat use.
Our study sites were focused at wetlands on Oahu Island, specifically two locations within the Pearl Harbor wetland complex, Āhua Reef Mitigation Wetland (Āhua Reef) and Pouhala Marsh Wildlife Sanctuary (Pouhala Marsh), and a site in Kaneohe at Marine Corp Base Hawaii (MCBH) on the eastern side of Oahu. Āhua Reef (21°19'01.6"N 157°57'28.3"W) is a mitigation wetland adjacent to the Honolulu international Airport. The Pouhala Marsh (21°22'38.5"N 158°00'22.0"W) is a state managed natural perennial wetland. The Kaneohe Reclamation Treatment Facility (21°26'17.3"N 157°45'30.2"W) is a wastewater treatment facility for the Marine Corp Base, adjacent to a series of freshwater wetlands. Unlimited invertebrate food sources are considered to be one of the factors that attract waterbirds at wastewater treatment ponds.
Hawaiian stilts were caught using decoy lures and 16 x 32 ft dropnets (Doherty 2009). Captured stilts were marked with unique color leg band combinations consisting of a Federal Bird Band and three plastic color bands to form a unique color combination. Hawaiian stilts were outfitted with Lotek Pinpoint GPS solar satellite tracking tags, attached using a leg-loop harness design with ¼ inch Teflon ribbon and a rubber band weak-link design. GPS Satellite tags were programed to log a point once every 1.5 hours from 4AM to 8 PM daily.
In ArcGIS, habitat factors were quantified by categories of: habitat type [4 categorical factors: Developed Areas, Undeveloped Fields, Sports Fields and Wetland Habitats], level of protection [2 categorical factors: Protected and Unprotected] and scale of anthropogenic development [4 categorical factors: Natural, Semi-Natural, Semi-Urban, and Urban]. Habitat type categories were determined by reviewing most recent aerial imaging in ArcMap and visually determination of the type of habitats at the GPS point logged. Anthropogenic development was determined by using a modification of the SmartCode developed by Duany and Talen (2003) and visually looking at the points on the aerial imagery.
Hawaiian stilt in Mo’olelo
In Hawaiian lore, the Hawaiian stilt or Ae’o is a kinolau, manifestation, of the Hawaiian god Kū, in his fisherman form. The Hawaiian stilt shares its name with the Hawaiian dry forest tree ‘Ohe makai or ‘Ohe kūkuluae’o, Reynoldsia sandwicensis, a tree used to make the stilts for Hawaiian dance. Kūkuluae’o the alternative name for the Hawaiian stilt translates to “the stilted pillar”, as Hawaiian stilts were considered to be the foundations of Hawaiian wetland systems and were honored with this distinction by Native Hawaiians.
Behavior and Ecology
Hawaiian stilts nest in mudflats, or rain-fed and tidal marshes when preferred habitats are inundated with water. These perrenial marshes become important habitats for waterbirds in these high rain events, increasing foraging outside of core wetlands and attract waterbirds from outside the habitat to establish themselves. The presence and abundance of Hawaiian stilts is dependent on the water levels and salinity of their habitats. Citizen scientists on O’ahu have observed Hawaiian stilt banded on other islands during bi-annual bird counts and incidental sightings throughout the year outside of core wetland habitats.
Our analysis of habitats locations collected indicated that Hawaiian stilts use nontraditional urban and semi-natural habitat types, such as sports fields and developed areas at higher frequencies in the evening when compared to natural wetland habitats. We observed 50% of locations in developed areas, 47% of locations in wetland habitats, 28% of locations in sports fields and 22% of locations in undeveloped fields in the evening. Of these habitat types, we observed 35% of locations were urban, 29.5% were in natural and semi-natural, and 6% in semi-urban sites.
The time of day in which Hawaiian stilts were at a given habitat varied. Natural wetland habitats had high occupancy throughout the day but peaking between 7-10 am. Between 7-10, proportion of locations in wetlands habitats were above 70%, but other times the average occurrence in wetland habitats were 10% of the overall habitat occurrence. In the late afternoon, wetland habitat occupancy decreased to 50% of the overall location proportion and continued to decline into the evening to less than 12% of the habitat occurrence. Occupancy for atypical habitats semi-natural and urban showed an increase beginning in the mid afternoon.
Observations of movement showed a pattern of habitat use previously undocumented by other Hawaiian stilt observation studies, mainly the use of atypical urbanized habitats. The high uses of semi-urban and urban habitats throughout the day of these birds, suggesting a level of adaption to anthropogenic landscape like that of O’ahu. It's concerning is that a high number of the sites outside of wetlands are unprotected, exposing birds to predation, vehicle strikes, and other threats not found within wetland habitats. Hawaiian stilts are a species that likely will need to be managed in perpetuity (Underwood et al. 2013). In the changing environment, traditional wetland habitats are increasing becoming scarce, especially on the island of O’ahu which has seen an increase in development. Global sea level rise is expected to adversely affect the state of Hawaii and likely will reduce the amount of coastal habitat for Hawaiian stilts, increasing the need for protected upland habitats. These results, although preliminary, could aid Hawaiian waterbird managers in supporting the increase of management outside of traditional wetlands and view atypical sites as potential habitat for Hawaiian stilts working with the county of Honolulu towards supportive management.
These temporal shifts in habitat use could be a response to anthropogenic light or lunar phases. Anthropogenic lights have been shown to influence nocturnal movements and activities of some passerines and waterbird species (Waston et al. 2016; Cabrera-Cruz 2018). Hawaiian stilts are opportunistic feeders and potentially could be using the increase in lights during new moons to increase feeding opportunities outside of wetland habitats. Furthermore lunar phases are linked to tidal influx and stilts will use exposed tidal flats as habitat and to forage. Seasonal rain patterns could likely influence movements, undeveloped fields after heavy rains likely look much like wetlands to Hawaiian stilts from above. The use of GPS satellite solar tracking tags can be used to investigate the influences anthropogenic light, lunar phase and seasonal weather patterns potentially have in the urban habitat use of Hawaiian stilts.
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1940 photo of Diamondhead © Bishop Museum
Hawaiian stilt chick © Amanda Sandor
Level of Anthropogenic development © Center for Applied Transect Studies
Adult stilt with chick © Hugo de Vries
We thank the U.S. Geological Survey, NavFac Hawaii, UH Hilo Hawaiian Cooperative Studies Unit, U.S. Forest Service for their support of this work. We are additionally indebted to the Hawaii Division of Forestry and Wildlife, Marine Corp Base Kaneohe for helping us coordinate field research and manage logistics. I personally thankful for the field assistance provided by Eben Paxton, Amanda Sandor, Hays Paxton, Marcom Gomes and Todd Anderson.