Graduate Thesis Or Dissertation

Movement Ecology and Population Dynamics of the Endangered Hawaiian Duck (Anas wyvilliana)

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  • Fundamental objectives in the field of conservation biology involve understanding the processes that influence small and declining populations and applying that knowledge to develop appropriate monitoring strategies and targeted management and conservation actions. Critical first steps in determining the relative role of factors that drive population declines involves estimation of key demographic rates, as well as movement parameters needed to provide the spatiotemporal context for demographic processes. Whereas traditional studies in avian ecology have focused on single phases of the annual cycle, advances in technology now permit researchers to track individuals throughout the year and thus account for linkages between various life history phases. This full annual cycle approach provides a more comprehensive framework for understanding movement ecology and population dynamics and, thus, identifying factors limiting recovery of species at risk. The Hawaiian Duck (Koloa maoli, Anas wyvilliana) is a non-migratory dabbling duck and the only extant endemic duck in the main Hawaiian Islands of more than twelve Anatids found in the fossil record. Once common throughout the Hawaiian Islands, Hawaiian Ducks experienced substantial population declines during the early 20th century and are currently restricted to the islands of Kaua‘i and Ni‘ihau. Similar to many endemic island bird populations, Hawaiian Duck population persistence on Kaua‘i is thought to be threatened by introduced predators, pathogens, and habitat loss and alteration. However, literature on Hawaiian Duck ecology is limited, and demographic estimates needed to assess population health, determine the relative importance of threats, and inform conservation planning are lacking. My dissertation aims to apply a full annual cycle framework to examine the influence of several biological and environmental factors on the movements, space use, and survival of the Hawaiian Duck and to provide critical baseline estimates of demographic parameters (e.g., abundance, sex ratio) needed for management and conservation planning. In Chapter 2, I employed a multi-spatiotemporal strategy using satellite and radio-telemetry to investigate large-scale regional movement patterns and to evaluate how sex, climate, and biological seasons influence regional fidelity and local-scale home range dynamics throughout the full annual cycle. Birds exhibited multiple regional movement strategies, including intra-island movement, inter-island movement (Kaua‘i–Ni‘ihau), and year-round residency. I documented large-scale movement (>80 km); however, most birds exhibited resident behavior and strong regional fidelity to north-central Kaua‘i on a seasonal and annual basis. Regional fidelity was higher among females than males, and higher during the peak nesting season (September–May) compared to the off-peak nesting season (June–August). On a seasonal basis, females ranged over slightly larger areas during wetter seasons compared to drier seasons; however, females generally maintained high inter-seasonal fidelity to home ranges and core use areas. Combining sexes, population-level hotspots (i.e., significant clustering of overlapping annual core use areas) occurred exclusively within Hanalei National Wildlife Refuge, where year-round wetland management specifically targets Hawaiian Duck. The combination of strong regional fidelity and extensive use of Hanalei NWR when on the north shore suggests this refuge supports a large proportion of Hawaiian Ducks for their full range of daily and seasonal life history requirements. Overall, these results provide insight into the spatiotemporal scale required for conservation and management actions. In Chapter 3, I used radio-telemetry data to evaluate factors influencing adult survival and to identify primary sources of mortality for Hawaiian Ducks throughout the annual cycle. Additionally, I used contemporaneous Hawaiian Duck carcass recovery and surveillance data to examine temporal and climatic associations with avian botulism outbreaks at a core wetland site, Hanalei NWR. Bi-monthly survival decreased log-linearly with lagged rainfall and did not vary with sex or peak nesting season. Annual survival for Hawaiian Ducks (62–80%) was relatively high compared to continental anatids. Primary causes of mortality included avian botulism and depredation by cats (Felis catus). The botulism surveillance dataset revealed weak support for the effect of rainfall on the number of sick and dead birds recovered, with generally a greater number of recoveries during months with middle-range concurrent-lagged rainfall totals. These results provides critical demographic data for population monitoring and highlight the importance of managing botulism risk and non-native mammalian predators for the recovery of the Hawaiian Ducks. In Chapter 4, I used a 5-year capture-mark-recapture (CMR) dataset and robust design CMR models to estimate adult abundance, sex ratio, and survival of Hawaiian Ducks at Hanalei NWR. During 2010–2015, 792 Hawaiian Ducks were captured 1,236 times. Model-averaged sex-specific abundance estimates revealed a heavily distorted, male-biased sex ratio (3:1) that was relatively consistent across seasons and years. Annual apparent survival was lower for females (0.43) compared to males (0.69). I found strong evidence for the presence of temporary emigration, but the particular form of emigration (i.e., random vs. Markovian) was unclear. The top model suggested random temporary emigration varied on a seasonal basis, with a greater proportion of birds moving to an unavailable state during summer compared to winter. Abundance estimates of Hawaiian Ducks at Hanalei NWR varied from 252 to 625 adult and first-year birds; however, these estimates reflect the abundance of birds available for capture, which may underrepresent the total number of birds on the refuge. This chapter provides estimates of several demographic parameters critically needed for improving our understanding of Hawaiian Duck population dynamics and informing future modeling efforts and conservation decisions. Collectively, these results provided novel insight into the movement ecology and population demography of the endangered Hawaiian Duck and could be applied to future modeling efforts, population assessments, and conservation planning. This research indicated that a large number of birds can be supported on managed sites, such as Hanalei NWR, throughout all phases of the annual cycle, suggesting that intense conservation of relatively small, protected lands can be an effective management strategy. Avian botulism was the primary cause of mortality and posed a year-round threat to Hawaiian Ducks. Combined with strong regional fidelity of birds to north Kauaʻi, these results highlight the importance of establishing a network of core wetlands to alleviate potential demographic bottlenecks. Further, the adult sex ratio was strongly male-biased and associated with lower survival among females than males over a 5-year span. Consideration of this important element of population structure is critical for future modeling efforts and population assessments. Lastly, these results demonstrated the potential of CMR methods as one component of a future long-term monitoring strategy.
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