Graduate Thesis Or Dissertation
 

Reproductive success of the Hawaiian common gallinule (Gallinula galeata sandvicensis) nesting in taro and managed wetlands on Kaua‘i, Hawai‘i

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/4m90f399t

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  • The endangered Hawaiian common gallinule (Gallinula chloropus sandvicensis) is an endemic Hawaiian waterbird that occurs only on O‘ahu and Kaua‘i. Hanalei National Wildlife Refuge (NWR) supports the largest population of Hawaiian gallinule on either island and is also one of the highest producers of commercially grown taro in the state. Effective management of Hawaiian gallinule at this site is hindered by a lack of information about the causes and timing of nest loss and chick mortality and how those demographic parameters differ between the two main wetland habitats on the refuge, taro and managed wetlands. I used cameras and radio telemetry to estimate nest and chick survival and compare both parameters between taro and wetlands during 2020 and 2021 (n = 162 total nests) on Hanalei NWR. I also determined habitat characteristics associated with nest success and collected data to test if cameras influenced nest success. I found no consistent effect of cameras on nest survival and their use improved our ability to understand the factors causing nest loss. Daily nest survival was 0.979 (95% CI: 0.972 - 0.984) and nest survival across the 25-day incubation period was 0.588. Nest survival, clutch size, and hatching success were similar between taro and managed wetlands and farming practices had little effect on nest survival. Predation during incubation were low (6% of failures), and thus, nest type, canopy cover, vertical obstruction, distance of nest from the nearest bank, and harvest status had negligible impact on nest success. Peak discharge of the Hanalei River was the most important driver of nest survival, as nest success declined due to flooding during periods of high river flows. Abandonment was the most frequent cause of nest failure (73% of failures), but the reasons for abandonment were largely unknown. While the effect of the water drawdown on nest survival in the analysis of all nests was negligible, I found that a disproportionate number of nests failed due to abandonment or predation on days when water had been drawn down. Several nests were flooded and subsequently failed as a direct or indirect consequence to water level manipulations by taro farmers, and one nest was accidentally destroyed during lo‘i maintenance. Feral pigs (Sus scrofa), and rats (Rattus spp.) depredated nests in both taro lo‘i and wetlands during incubation. The absence of feral cat depredation in my study may indicate that the ongoing efforts to trap and remove feral cats at this site are successful at minimizing predation of Hawaiian gallinule nests. My study presents the first estimate of chick survival of Hawaiian gallinule from observations of marked adults with their young and confirms the low probability of chick survival to fledgling. Chick survival was 0.18 ± 0.08 from hatch to 40 days but chick survival was lower in the first half of the 40-day, pre-fledging period (0.78 ± 0.05 per 4-day interval) than the second half (0.88 ± 0.09). I documented 4 barn owl (Tyto alba) attacks on chicks and adults in nests within 7 days post-hatch when broods were roosting at night, and one depredation of a chick by a black-crowned night heron (Nycticorax nycticorax). Camera monitoring revealed that gallinule broods utilize nests for roosting longer than previously thought, and chick survival is lower in the first half of the pre-fledging period. Consequently, nests should be protected from disturbance during this critical time. My results suggest that despite a nest survival rate that is adequate for population growth and the ability of gallinule to nest multiple times a year, the low rate of chick survival is likely limiting the reproductive potential of Hawaiian gallinule at this site. Barn owls may be an important predator of gallinule chicks, but more research is needed to determine cause-specific chick mortality and the impact of taro farming practices such as weeding and water drawdowns on gallinule chick survival. Without island-wide solutions to the threat of invasive predators, the recovery and persistence of gallinule populations will require predator control efforts in perpetuity. My research will contribute to collaborative statewide conservation planning efforts designed to facilitate the recovery of the species by providing important productivity parameters and insight on habitat relationships in a core wetland on Kaua‘i.
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