A University of Hawai‘i team have found that fencing to exclude mammals improves the nesting success of aeʻo (Hawaiian Stilt). Researchers used motion-activated game cameras to monitor nests at two sites on O‘ahu, one with mammal-exclusion fencing and one without. Clutch sizes (the number of eggs laid per nest) and hatch rates were significantly greater at the fenced site than the unfenced site.
The results add to the mounting body of evidence showing bird populations responding well to the removal of mammal predators. Hawaiian waterbirds evolved without mammals and they are largely defenseless against non-native feral cats, rats and mongoose. The fencing resulted in a positive breeding response even though it did not exclude bullfrogs or aerial predators such as the introduced Barn owl, highlighting that invasive mammals are likely the single greatest predatory threat to the Hawaiian Stilt.
The research was conducted at two wetland units within the Pearl Harbor National Wildlife Refuge Complex: Honouliuli and Waiawa. The two sites have similar rainfall regimes and proximity to urban development, while the salinity and plant composition varied slightly.
In August 2018, a 1,006 meter mammal-exclusion fence was built at Honouliuli by the U.S. Fish and Wildlife Service, with one side open to the estuary. The fence is based on the Xcluder™ Kiwi model and effectively kept out mammals. Other potential waterbird predators remain a threat, however, and more research is needed to understand their impacts on the waterbird populations.
The research was conducted by Dain Christensen, Kristen Harmon and Nathaniel Wehr, under the guidance of Dr. Melissa Price. The team noted that while mammal-exclusion fencing is initially expensive, it can be cost effective over the lifetime of the structure. The research indicates that predator-proof fencing will be an important conservation tool on the Hawaiian Islands, not just for the aʻeo but all of Hawaiʻi’s endemic waterbirds.
As habitat for these species is lost to sea level rise, it is increasingly important to direct our attention to the most effective conservation actions. Increasing nesting success for the endangered aʻeo by excluding non-native mammals matches up with that goal.