The Hawaiian Stilt or Ae'o has both cultural and ecological value. Listed as endangered in 1967 under the Endangered species act (ESA), the U.S. Fish & Wildlife Service requires a population of 2,000 individuals to delist the species. To grow a self-sustaining population, active predator control has been recommended until non-native predators are eradicated. Stilts are precocious and it is unclear precisely how soon they leave the nest area after hatching. Freshly hatched chicks are highly vulnerable to predation. Estimates of chick mortality and homerange during this vulnerable period are essential to maximizing fledging success and increasing population stability.
Season I objectives: Camera Traps
Season II objectives: VHR Radio Transmitters
Figure 1: Study sites illustrated above. MCBH was excluded from season one because authors did not have access to the base.
<10 days old: Motion-activated cameras
Sites were surveyed weekly to identify nests, track the time of hatching of eggs, and age of chicks. Bushnell Trophy game cameras were placed on posts two meters away and one meter above the water level looking down into the nests prior to hatching. Cameras were set to take a photo at dawn, dusk, once instantaneously after being triggered and then again after a two-second delay. Pictures are being reviewed to identify predators and chick survival at the end of each day.
Total nests monitored:
12 nests monitored 37 chicks.
Total run time after hatching: 3,391 hours (Min-Max: 18 -576) Mean: 226 hours (9.42 days).
Reappearance of chicks at the nest site: 9/12 nests did not have chicks reappear into the frame of the camera trap. Nine of the reappearances came from three nests inside the Waiawa PHNWR unit.
Predators: <12 minutes of predators captured in the frame from 3,391 hours of data.
Leaving the nest: Chicks left the nest on average 51.5 hours after the first chick had hatched. Time spent at the nest ranged from 18 to 79 hours (n=4).
The use of camera traps for mark and recapture data on stilt chicks is not practical. Chicks reappeared into the frame of the camera infrequently and the probability of recapture data is very low.
A definitive definition of "leaving the nest," is necessary to quantify how long chicks remain in the nest area. Chicks in the frame of the camera trap were considered to be in the nesting area for season one of this study. The area in the camera trap frame around the nests was undefinable and inconsistent.
Due to the nature of camera traps, Cattle Egrets where the predominant predators triggering the camera.
The Pearl Harbor Honouliuli unit is in progress of having a predator-proof fence erected, allowing a rare opportunity to run experiments comparing the Honouliuli unit with other O‘ahu wetland systems and analyzing the impact of native and non-native predators Ae‘o chicks.
Steps moving forward include repeating 2018 data collection on chicks < 10 days old, increasing the total sample size of nests in camera trap data, defining a consistent definition of the nesting area, and implementing season II methods described below.
>10 days old: Banding and transmitters
Chicks 10 days of age or older will be randomly captured by a butterfly net and biometrics measured. Each chick will be banded with a standard US Geological Survey aluminum band, as well as size 4 (6.35 mm internal diameter) PVC leg rings with two alpha-numeric codes to identify individuals. 0.75g (< 3% body mass) A2435 transmitters from Advanced Telemetry Systems Inc. with a battery life of 60 days at 24 pulses per minute (ppm) will be glued directly on to the skin 1cm above the preen gland with “Locktite” superglue. If a transmitter becomes detached from a chick, the alpha-numeric codes will be used to identify individuals and their locations estimated. Chicks will be hand tracked daily, locations determined by biangulation and visually located as often as possible. If a chick was suspected to have died, it was located within 24 hours to determine the cause of death.
Figure 1. Dorsal view showing attachment site for radio transmitters.
Figure 2. Advanced Telemetry Systems VHF A2400 series.
Special thanks to Kimber Troumbley and Melissa Jones. Also, thank you to the Wildlife Ecology Lab and the Department of Natural Resource and Environmental Management. Thank you to the USFWS and DOFAW for the use of their sites during the first season of this study.