Maunalua Bay Stream Retrofitting Report

Analysis of potential retrofitting options for streams in order to reduce sedimentation loads entering the Maunalua Bay, Oahu. We conclude that this is a system-wide issue that cannot be solved with retrofitting alone.Malama Maunalu

Mammal-exclusion fencing improves the nesting success of an endangered native Hawaiian waterbird

Invasive predator control is often critical to improving the nesting success of endangered birds, but methods of control vary in cost and effectiveness. Poison-baiting or trapping and removal are relatively low-cost, but may have secondary impacts on non-target species, and may not completely exclude mammals from nesting areas. Mammal-exclusion fencing has a substantial up-front cost, but due to cost savings over the lifetime of the structure and the complete exclusion of mammalian predators, this option is increasingly being utilized to protect threatened species such as ground-nesting seabirds. However, non-mammalian predators are not excluded by these fences and may continue to impact nesting success, particularly in cases where the fence is designed for the protection of waterbirds, open to an estuary or wetland on one side. Thus, there remains a research gap regarding the potential gains in waterbird nesting success from the implementation of mammal-exclusion fencing in estuarine systems. In this study, we compared the nesting success of endangered Hawaiian Stilts (Ae‘o; Himantopus mexicanus knudseni) within a mammal-exclusion fence to that of breeding pairs in a nearby wetland where trapping was the sole means for removing invasive mammals. We predicted success would be greater for breeding pairs inside the exclusion fence and the hatchlings inside the enclosure would spend more time in the nesting area than hatchlings at the unfenced site. During a single breeding season following construction of a mammal-exclusion fence, we used motion-activated game cameras to monitor nests at two sites, one site with mammal-exclusion fencing and one site without. Clutch sizes and hatch rates were significantly greater at the fenced site than the unfenced site, but time spent by chicks in the nesting area did not differ between sites. These results add to the mounting body of evidence that demonstrates the effectiveness of mammal-exclusion fencing in protecting endangered birds and suggests it can aid endangered Hawaiian waterbirds toward recovery. These results also suggest that the single greatest predatory threat to the Hawaiian Stilt may be invasive mammals, despite a host of known non-mammalian predators including birds, crabs, turtles, and bullfrogs, as the complete exclusion of mammals resulted in significant gains in nesting success. As additional fences are built, future studies are necessary to compare nesting success among multiple sites and across multiple seasons to determine potential gains in fledging success and recruitment

Mammal-Exclusion Fencing and the Reproductive Success of an Endangered Native Waterbird

Novel relationships in ecological communities are forming faster than historical rates due to globalization and the resulting increase in species introductions. In the Hawaiian Islands, which prior to humans had no terrestrial reptiles or amphibians and only one terrestrial mammal, the introduction of invasive predators dramatically impacted island food webs. Wetlands, as ecosystems where terrestrial, aquatic, and marine species intersect, were particularly impacted by introduced species. The Hawaiian Stilt (Himantopus mexicanus knudseni) or Ae‘o, is one of five Hawaiian waterbirds listed under the Endangered Species Act. Currently estimated to range from ~1,300 to ~1,800 individuals, the Hawaiian Stilt must reach a self-sustaining population of 2,000 birds in order to be delisted. One factor hindering recovery may be the predation of Stilt eggs and chicks by invasive predators. To address this threat, the U.S. Fish and Wildlife Service recently constructed a fence that excludes mammalian predators around a wetland on O‘ahu. Although most island avian communities respond positively to invasive mammalian predator removal, mammal-exclusion fencing is an expensive tool, and only controls for one type of predator – mammals. Avian, aquatic, and amphibious predators may still impact birds inside the conservation fencing. In this study, I compared the reproductive success of Stilts nesting inside and outside the newly built mammal-exclusion fence to test hypotheses regarding the impact of invasive predators and estimate the effectiveness of mammal-exclusion fencing as a management action. The results of the proposed research should help to inform management decisions regarding which predator control tools will be most cost-effective in a given scenario, by identifying the improvement in reproductive success of the Hawaiian stilts nesting inside versus outside of a mammalian predator exclusion fence

Nesting Ecology of the Hawaiian Stilt (Himantopus mexicanus knudseni) on O‘ahu

The Hawaiian Stilt (Himantopus mexicanus knudseni) is an endangered subspecies of the Black-necked stilt (Himantopus mexicanus) that inhabits wetlands throughout the Hawaiian Islands. Depredation of eggs and chicks by introduced predators is a major threat to Hawaiian Stilt populations. Where and when a bird decides to nest may impact the likelihood of egg or chick depredation. Nesting in close proximity to water may decrease depredation rates by mammals, as water can act as a barrier to mammalian predators, does not hold scent, and provides an obstacle-free escape route for chicks. Alternatively, some mammalian predators may be attracted to water, and a number of aquatic species have been identified as predators of Hawaiian Stilt chicks, including the American Bullfrog (Lithobates catesbeianus). Vegetation height is also an important factor for egg and chick survival, as taller vegetation may help conceal nests and chicks from predators, particularly aerial species. Additionally, depredation is often not constant across the breeding season due to changes in parental activity, nest and chick abundance, or habitat characteristics. The Hawaiian Stilt nests from February to September across the Hawaiian Islands. The nesting season coincides with a seasonal decline in precipitation, which may alter habitat characteristics and thus impact depredation rates. Further, management tools, such as mammal-exclusion fencing, are currently in use and may greatly increase egg and chick survival. The objectives of this project were to: 1) identify habitat characteristics important for nest-site selection and chick habitat use; 2) identify factors that impact hatching and fledging success. We found that stilts preferred to nest in shorter vegetation than what was available and preferred Pickleweed (Batis maritima) rather than other available plant species. However, nest-site characteristics, such as vegetation height and distance to water, did not have an impact on egg depredation risk. Early nests had a higher chance of hatching than late nests. The number of depredated nests peaked later in the nesting season, following a peak in nest initiation. Introduced mammals were the primary egg predators and included rats (Rattus spp.), feral cats (Felis catus), and Small Indian Mongooses (Herpestes auropunctatus). The number of eggs laid, as well as hatching success, was greater inside the mammal exclusion fence at Honouliuli Wetland, compared to a nearby site without a fence, Waiawa Wetland, where mammalian predators are only excluded via trapping. The average home range size for 12 tracked pre-fledglings was 0.94 ± 1.42 acres, and most chicks were observed using vegetated mudflats near open water. Of the 20 chicks that were tracked in this study, 7 fledged (35%), 6 had unknown fates (30%), 4 died due to unknown causes (20%), 2 were depredated by a feral cat (10%), and 1 died due to emaciation (5%). Our results suggest that management of predators, particularly mammals, is key to improving stilt hatching success, as preferred nest-site characteristics do not reduce the likelihood of egg depredation. Tall, invasive vegetation, such as California Grass (Brachiaria mutica), should continue to be controlled, as it was rarely used for nesting. More desirable vegetation, such as Pickleweed, should be made available throughout wetlands to encourage larger spacing between nesting pairs, which may help to reduce egg depredation pressure. Increasing mammalian predator control later in the nesting season may also increase hatching success of later nesters. Alternatively, mammal-exclusion fencing may provide year-round protection from mammalian predators, increasing both egg and chick survival. More data is needed to form conclusions regarding home range and survival of Hawaiian Stilt chicks. Improved detection methods and radio-tagging attachment styles will be used in the 2020 nesting season, which will reduce uncertainties and improve statistical power of analyses