Christensen Dain L, Harmon Kristen C, Wehr Nathaniel H, Price Melissa R
Department of Natural Resources and Environmental Management, College of Tropical Agriculture and Human Resources, University of Hawaii at Mānoa, Honolulu, Hawaii, United States of America.
PeerJ. 2021 Mar 1;9:e10722. doi: 10.7717/peerj.10722. eCollection 2021.
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; ) 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.
对入侵性捕食者进行控制通常对于提高濒危鸟类的筑巢成功率至关重要，但控制方法在成本和有效性方面存在差异。投放毒饵或诱捕并清除相对成本较低，但可能会对非目标物种产生次生影响，并且可能无法完全将哺乳动物排除在筑巢区域之外。哺乳动物排除围栏前期成本很高，但由于该结构在其使用期限内可节省成本，并且能完全排除哺乳动物捕食者，因此这种方法越来越多地被用于保护诸如地面筑巢海鸟等濒危物种。然而，这些围栏无法排除非哺乳动物捕食者，它们可能会继续影响筑巢成功率，特别是在围栏是为保护水鸟而设计且一侧通向河口或湿地的情况下。因此，关于在河口系统中实施哺乳动物排除围栏对水鸟筑巢成功率的潜在提升，仍存在研究空白。在本研究中，我们将濒危的夏威夷长脚鹬（Ae'o; ）在哺乳动物排除围栏内的筑巢成功率与附近湿地中通过诱捕作为清除入侵哺乳动物唯一手段的繁殖对的筑巢成功率进行了比较。我们预测，排除围栏内的繁殖对成功率会更高，并且围栏内的幼雏在筑巢区域停留的时间会比未设围栏地点的幼雏更长。在建造哺乳动物排除围栏后的一个繁殖季节里，我们使用运动激活式游戏相机监测了两个地点的巢穴，一个地点有哺乳动物排除围栏，另一个地点没有。围栏地点的窝卵数和孵化率显著高于未围栏地点，但幼雏在筑巢区域停留的时间在两个地点之间没有差异。这些结果进一步证明了哺乳动物排除围栏在保护濒危鸟类方面的有效性，并表明它有助于濒危的夏威夷水鸟恢复。这些结果还表明，尽管存在许多已知的非哺乳动物捕食者，包括鸟类、螃蟹、海龟和牛蛙，但对夏威夷长脚鹬最大的单一捕食威胁可能是入侵哺乳动物，因为完全排除哺乳动物使筑巢成功率有了显著提高。随着更多围栏的建造，未来有必要进行研究，比较多个地点和多个季节的筑巢成功率，以确定在幼鸟成功离巢和补充数量方面的潜在提升。
