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

Abstract

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.

Cooperative breeding behaviors in the Hawaiian Stilt (Himantopus mexicanus knudseni)

“…The manipulated stable conditions at the KWRF on Moloka'i may explain the cooperatively defensive behaviors of the group of related individuals toward unrelated individuals, although group defensive behaviors are common in semi-colonial breeders (Robinson et al, 2020). Alternatively, cooperative breeding in Hawaiian Stilts may also be a response to reduced reproductive success caused by introduced predators (van Rooij & Griffith, 2013;Sorato et al, 2012), as introduced predators are the major cause of nest failure in Hawaiian Stilts (Christensen et al, 2021;Harmon, Wehr, et al, 2021). Indeed, cooperative breeding behaviors by Hawaiian Stilts on O'ahu were observed in areas with high nest depredation by introduced Small Indian Mongooses, rats, and feral cats.…”

A prioritization protocol for coastal wetland restoration on Molokaʻi, Hawaiʻi

“…Furthermore, several of the top rated sites for bird habitat (OHI, PUNA, KNWR, KAMA, and PAHU) had low ratings for predator and botulism control due to a lack of management (Table 4). Although even simple ponds can provide habitat for endangered waterbirds (including wastewater treatment ponds containing raw sewage (KAUN)), unmanaged sites can act as 'sinks' with high mortality, especially for chicks, due to the abovementioned threats (Christensen et al, 2021;Harmon et al, 2021). The recovery of endangered waterbirds in Hawaiian wetlands depends on restoring and expanding highquality habitat combined with strong protections from predators, hazards, and diseases.…”

Behavioural differences in predator aware and predator naïve Wellington tree wētā, Hemideina crassidens.

“…In these island systems, as well as in other environments with high numbers of endemic species at risk of extinction, fenced ecosanctuaries or aggressive trapping programmes have reduced or eradicated invasive predators for the benefit of native species. These approaches thus provide an environment with a predator array closer to that before introduced species arrival ( Christensen et al., 2021 ; Ringma et al., 2020 ).…”