•Listed as Endangered in 1967, the U.S. Fish & Wildlife Service (USFWS) requires a population of 2,000 individuals to delist the subspecies (USFWS 2009).
•To grow a self-sustaining population, active predator control is required until non-native predators are eradicated.
•Freshly hatched chicks are highly vulnerable to depredation. Accurate estimates of chick mortality during this vulnerable period are essential for assessing the risk of extinction and increasing population stability (Reed 1998).
•The 0-10 day window is a very vulnerable time and it is suspected that many chicks are depredated or die reducing the overall number of chicks that can be found. By day 10, if they are still alive, chicks are far more mobile.
•Stilt response to predators differs depending on the predator type (Sordahl 2004).
•With limited resources, managers strive to make decisions that result in the greatest reduction in extinction risk for the lowest cost.
•The USFWS recently completed the construction of a predator-proof fence around the Honouliuli unit of the Pearl Harbor National Wildlife Refuge (PHNWR) in hopes to exclude a subset of predators from the refuge.
•This study examines the benefits of predator-proof fencing around Honouliuli and compares it to the Waiawa unit, an adjacent wetland site without predator-proof fencing.
•Prior to hatching, motion activated cameras were set three meters away and one meter high to detect Stilts in the nesting area.
•Both study sites were visited weekly to survey for new nests and to maintain the cameras on active nests.
•The Hawaiian Stilt breeding occurs predominantly from March to August and peaks between May to June (Coleman 1981). Data collection for the 2019 season is ongoing.
•After a nest had hatched, the camera was left up for at least 10 days. Nests that were predated or flooded had their camera taken down soon after.
• A portion of the data was processed and analyzed. Chi-squared analyses were used to test the difference in hatch rate and depredation rate among the two sites.
•There is a significant difference in the # of eggs hatched between Honouliuli and Waiawa for 2019.
p<0.001 X2=20.58 Chi-square
•There is a significant difference in the # of depredated nests between Honouliuli and Waiawa 2019.
p=0.05 Fisher's Exact Test (for small sample size) *Used Fisher Exact Test, so no chi-square value
•There is a significant difference in the # of depredated nests between Honouliuli 2018 and 2019.
p=0.02 X2=4.85 Chi-square
There is NOT a significant difference in the # of depredated nests between Honouliuli and Waiawa 2018.
p=0.19 X2=1.70 Chi-square
•There is NOT a significant difference among predator types in the proportion of days a potential predator visited each nest at Honouliuli in 2019.
p=0.34 F=1.11 ANOVA
In 2019, of the nests that were detected, Waiawa (without predator-proof fencing) had significantly fewer nests hatch when compared to Honouliuli (with predator-proof fencing). This may be due in part to heavy depredation by rats and cats . Of the nests detected at Honouliuli 2019, over 90% hatched successfully. This is a significantly higher proportion then Waiawa in 2019 but also Honouliuli itself in 2018, indicating improved reproductive success. It is unclear whether the preliminary results can be attributed to the predator-proof fencing or variance from year to year.
Novel interactions with introduced pressures such as rats and cats could drive novel responses in Hawaiian Stilts such as those depicted in Figure 4. Furthermore, based on the number of hatch-year sub-adults sighted throughout the island, some sites are suspected to be sinks for stilt chicks.
Bullfrogs are known predators of Stilt chicks and are detected more frequently at Honouliuli. This may be because it is more of a fresh water system when compared to the more saline Waiawa ponds. While predator-proof fencing may increase hatching success, it cannot be verified that there is increased recruitment into the population.
Preliminary results suggest that the predator-proof fencing erected by USFWS around the Honouliuli unit within PHNWR is in deed improving the reproductive output of Hawaiian Stilts. If true, this increase is one step closer to recovery for this conservation reliant species.
This research is impactful in 3 ways.
Special thanks to Kimber Troumbley, Melissa Jones, Clarine Phipps, Andreanna Cole, Jessica Idle, and Eryn Opie. Also, thank you to the Wildlife Ecology Lab and the Department of Natural Resources and Environmental Management at the University of Hawai‘i. Thank you to the USFWS and DOFAW for access to the sites and willingness to help during this study.
•Coleman, Richard Alan. 1981. “THE REPRODUCTIVE BIOLOGY OF THE HAWAIIAN SUBSPECIES OF THE BLACK-NECKED STILT, Himantopus mexicanus knudseni.” Pennsylvania State University. Pennsylvania State University.
•Reed, Michael, Chris S. Elphick, and Lewis W. Oring. 1998. “Life-History and Viability Analysis of the Endangered Hawaiian Stilt.” Biological Conservation 84 (1): 35–45. https://doi.org/10.1016/S0006-3207(97)00077-3.
•Sordahl, Tex A. 2004. “Field Evidence of Predator Discrimination Abilities in American Avocets and Black-Necked Stilts.” Journal of Field Ornithology 75 (3): 223–31. https://doi.org/10.1648/0273-8570(2004)075.
•USFWS. 2011. “Recovery Plan for Hawaiian Waterbirds: Second Revision.” U.S. Fish and Wildlife Service. Portland. https://www.fws.gov/pacificislands/CH_Rules/Hawaiian Waterbirds RP 2nd Revision.pdf.