At the far end of the northwestern Hawaiian Islands lies Kuaihelani (also known as Midway Atoll), a small island set that is home to the world’s largest albatross colony. More than a million albatrosses return to Kuaihelani each year to breed. These seemingly unspoiled islands appear safe, but there is a predator lurking among the seabirds.
House mice (Mus musculus) — the same species that may be in your home — has begun attacking and killing albatrosses, eating them alive as they sit in their nests. I am an ecologist studying the mystery behind these killer mice.
A hunter hiding in plain sight
Kuaihelani, which was the scene of intense fighting during World War II, is now a national wildlife sanctuary.
Without predators such as cats, rats or mangustas, Kuaihelani is a place where mōlī (Phoebastria immutabilis), also known as Laysan albatrosses. Each about the size of a goose, these seabirds nest in almost the same spot each year and produce only one egg per year.
During the winter nesting season in 2015, volunteer bird counters and biologists began seeing horrific, 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 following weeks, they found dozens of injured mōlī, then hundreds.
Biologists were baffled. Had a groundhog escaped from a docked boat? Had a falcon flown away in the recent winter storm? Desperate to identify the culprit, biologists set up game cameras around the nesting mōlī.
Cameras have captured bizarre nighttime footage of mice chewing and crawling on mōlī’s backs and heads. This is the first time a house mouse has been observed attacking a live adult, nesting albatross.
Mōlī, like many seabirds, evolved on remote islands without predators. As a result, such seabirds are often oddly fearless and curious—pulling at researchers’ shoelaces or gnawing at our dashboards. This phenomenon is called “island naivety,” and, no matter how appealing, can be disastrous when non-native predators like rats and cats are introduced to islands. Lacking innate caution, even the largest seabirds can become vulnerable prey to predators as small as mice.
Improve meat taste
During World War II, the Kuaihelani islands were cleared and covered with wartime infrastructure, during which time both black rats and house mice were inadvertently introduced. Soon after, the rats began to decimate populations of nesting seabirds.
When Kuaihelani’s military importance 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. Considered small and harmless, they were of little concern until 2015.
Scientists may never know exactly why mice attack and kill mōlī, but we have some ideas.
Due to climate change, Kuaihelani has experienced increasingly erratic rainfall, sometimes resulting in long periods of drought or heavy downpours. During dry periods, vegetation dies rapidly. The usual food supplies for mice, such as seeds and insects, are likely to diminish during these periods. In order for mice to survive, they must find a different food source.
On an island home to millions of birds, seabird carcasses are plentiful and attract a rich assortment of insects, including cockroaches, isopods and worms. It turns out that rats have a strong appetite for these creatures and also feed on seabird carcasses. It’s only a short step from scavenging on dead seabirds to attacking live ones that don’t fight back.
As rat attacks on nesting mōlīs increased from 2015 onwards, it was clear that something had to be done – and soon. The solution was to get rid of the rats, which is unfortunately much easier said than done.
Stubborn mice
Mouse extermination is a difficult and risky conservation effort that requires years of research and careful planning. Ideally, rodenticide, a type of poison used to kill rodents, should be offered when the mice are hungriest and most likely to eat it. This requires knowing exactly what they are eating and when those food sources are scarce.
By extracting and sequencing DNA from mouse feces and analyzing stable isotopes—a technique that identifies the unique chemical fingerprints of organisms—my colleagues and I were able to understand which organisms the mice were eating and in what quantities. We found that the mice on Sand Island in Kuaihelani ate mostly insects (about 62 percent of their diet), followed by plants (27 percent), and finally albatross (probably mōlī, about 12 percent). The Fish and Wildlife Service has identified July as the best time to attempt eradication because seabird densities are usually at their lowest.
Due to COVID-19 disruptions, the eradication effort was delayed until July 2023, when the nonprofit Island Conservation and the Fish and Wildlife Service meticulously applied the rodenticide in multiple rounds. It seemed to work at first. But a few mice were seen in the weeks that followed — then more. By September 2023, the eradication was declared a failure.
While some conservationists believe eradication should be attempted again, others are concerned that rodents are becoming resistant to rodenticides. When rodents are repeatedly exposed to rodenticides over generations, they may begin to carry genetic mutations that result in resistance to the poison, rendering future eradication efforts ineffective.
There is no doubt that rats on Kuaihelani have been exposed to rodenticide for a long time. When Kuaihelani—or Midway Atoll—was a naval base, rodenticide was likely applied to buildings and residences. The 1996 rat extermination event was another exposure. I am currently investigating whether rats on Kuaihelani have these genetic mutations.
Concerns about rodent-resistant mice are not limited to Kuaihelani. Cases of rodent resistance are increasing worldwide, particularly in Europe. Rodents continue to have serious and widespread ecological impacts on islands around the world.
For now, I’m focused on helping mōlī survive in Kuaihelani, but our research could also shed light on the growing challenge of resilient mice around the world.
This article is republished from The Conversation, a nonprofit, independent news organization that brings you facts and trusted analysis to help you understand our complex world. 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. He is affiliated with the University of Tennessee, Knoxville, and collaborates with the U.S. Fish and Wildlife Service and Island Conservation.