At the tip of the Northwestern Hawaiian Islands lies Kuaihelani—also known as Midway Atoll—a small set of islands that are home to the world’s largest albatross colony. More than a million albatrosses return to Kuaihelani each year to breed. These seemingly pristine islands may seem safe, but a predator lurks among the seabirds.
House mice (Musculus muscle) — the same species that may be in your home — have begun attacking and killing albatrosses, eating them alive as they sit on their nests. I am an ecologist studying the mystery behind these murderous mice.
A predator hiding in plain sight
Once the scene of fierce warfare during World War II, Kuaihelani is now a national wildlife refuge.
Without predators such as cats, rats or mongooses, Kuaihelani provides a safe haven for millions of breeding and migratory birds, including mōlī (Phoebastria immutabilis), also known as Laysan albatrosses. These seabirds, each about the size of a goose, nest in almost exactly the same spot every year and produce only one egg per year.
During the winter breeding season of 2015, volunteers and biologists counting birds began seeing gruesome bloody wounds on nesting mōlī. At first, they found only a few mōlī with these mysterious injuries, which included severe chewing along the neck and even scalping. Over the next few weeks, they found dozens of injured mōlī, then hundreds.
Biologists were puzzled. Had a black rat escaped from a moored boat? Had a peregrine falcon flown in during the last winter storm? In a desperate attempt to identify the culprit, biologists placed trail cameras around nesting mōlī.
The cameras captured bizarre nighttime footage of mice crawling and chewing on the backs and heads of mōlīs. It was the first time a house mouse had attacked a living adult nesting albatross.
Mōlī, like many seabirds, evolved without predators on remote islands. As a result, such seabirds are often strangely fearless and curious—tugging researchers’ shoelaces or nibbling our clipboards. This phenomenon is called “island naiveté,” and while charming, it can spell disaster when nonnative predators such as rats and cats are introduced to islands. Lacking innate caution, even the largest seabirds can become defenseless prey to predators as small as mice.
Developing a taste for meat
During World War II, the islands of Kuaihelani were cleared and covered with war infrastructure. Both black rats and house mice were inadvertently introduced during this time. The rats soon began to decimate the populations of burrowing seabirds.
When the military importance of Kuaihelani declined in the 1990s, management of the atoll was transferred to the U.S. Fish and Wildlife Service. Rats were successfully eradicated in 1996, but mice remained. They were considered small and harmless and did not cause much concern until 2015.
While scientists may never know exactly why mice began attacking and killing mōlī, we do have a few ideas.
Due to climate change, Kuaihelani is increasingly experiencing irregular rainfall, sometimes resulting in long dry spells or heavy downpours. During dry spells, vegetation dies off rapidly. It is likely that the usual food items for mice, namely seeds and insects, decrease during these periods. In order to survive, mice must find another source of food.
On an island with millions of birds, seabird carcasses are plentiful and attract a rich community of insects, including cockroaches, woodlice and maggots. Mice seem to have a considerable appetite for these creatures and probably feed on seabird carcasses at the same time. The transition from preying on dead seabirds to attacking live seabirds that don’t fight back is a small step.
When mouse attacks on nesting mōlīs increased from 2015 onwards, it was clear that something had to be done – and fast. The solution was to get rid of the mice, which is unfortunately much easier said than done.
Die-hard mice
Eradicating mice is a challenging and risky conservation operation that requires years of research and careful planning. Ideally, rodenticide, a type of poison used to kill rodents, should be offered when mice are most hungry and likely to eat it. This requires knowing exactly what they eat and when those food sources are scarce.
By extracting and sequencing DNA from mouse feces and analyzing stable isotopes—a technique that identifies unique chemical fingerprints of organisms—my colleagues and I were able to determine which organisms were eating the mice and in what quantities. We found that mice on Sand Island of Kuaihelani primarily ate insects (about 62 percent of their diet), followed by plants (27 percent), and finally albatrosses (likely mōlī, about 12 percent). The Fish and Wildlife Service identified July as the best time to attempt the eradication, when seabird densities are typically lowest.
Due to disruptions caused by COVID-19, the eradication effort was delayed until July 2023, when the nonprofit Island Conservation and the Fish and Wildlife Service painstakingly applied rodenticide in multiple rounds. At first, it seemed to work. But in the weeks that followed, a few mice were spotted—and then more. In September 2023, the eradication was declared a failure.
Some conservationists believe eradication should be attempted again, but others worry about creating rodenticide-resistant mice. When generations of rodents are repeatedly exposed to rodenticide, they may carry genetic mutations that result in resistance to the poison, rendering future eradication efforts ineffective.
Without a doubt, mice on Kuaihelani have had long-term exposure to rodenticide. When Kuaihelani—or Midway Atoll—was a naval base, rodenticide was likely used in and around buildings and homes. The rat eradication in 1996 was another exposure. I am currently investigating whether the mice on Kuaihelani already have these genetic mutations.
Concerns about rodenticide-resistant mice are not limited to Kuaihelani. Around the world, particularly in Europe, there are increasing numbers of rodents carrying resistance. Rodents continue to have serious and widespread ecological impacts on islands worldwide.
For now, I’m focused on helping the mōlī of Kuaihelani survive. But our research can also help address the growing challenge of resistant mice around the world.
This article is republished from The Conversation, a nonprofit, independent news organization that brings you facts and reliable analysis to help you understand our complex world. It was written by: Wieteke Holthuijzen, University of Tennessee
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Wieteke Holthuijzen has received research funding from the National Science Foundation Graduate Research Fellowship Program, Northern Illinois University, Sigma Xi, and Island Conservation. She is affiliated with the University of Tennessee, Knoxville and has worked with the US Fish and Wildlife Service and Island Conservation.