At the tip of the Northwestern Hawaiian Islands lies Kuaihelani – also known as Midway Atoll – a small cluster of islands that is home to the world’s largest albatross colony. More than a million albatrosses return to Kuaihelani every year to breed. These seemingly pristine islands seem safe, but a predator lurks among the seabirds.
House mice (Mus musculus) – the same species that may be in your home – have started attacking and killing albatrosses, eating them alive while they are on their nest. I’m an ecologist studying the mystery behind these murderous mice.
A predator that hides in plain sight
Kuaihelani was once the scene of intense warfare during World War II and is now a national nature reserve.
With no predators such as cats, rats or mongooses, Kuaihelani provides a safe haven for millions of nesting and migratory birds, including mōlī (Phoebastria immutabilis), also called 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 annually.
In the 2015 winter breeding season, bird count volunteers and biologists began seeing gruesome bloody wounds on nesting mōlī. At first they found only a few mōlī with these mysterious injuries, including severe chewing on the neck and even scalping. In the weeks that followed they found dozens of injured mōlī, then hundreds.
Biologists were astounded. Had a black rat escaped from a moored boat? Did a peregrine falcon fly by during the last winter storm? Desperate to identify the culprit, biologists set up wildlife cameras around nesting mōlī.
The cameras captured bizarre nighttime images of mice crawling and chewing on the backs and heads of mōlī. It was the first time a house mouse had ever attacked a living adult nesting albatross.
Mōlī, like many seabirds, evolved without predators on remote islands. As a result, such seabirds are often strangely unafraid and curious: they pull on the shoelaces of researchers or nibble on our clipboards. This phenomenon is called ‘island naivety’ and, while charming, can spell disaster if non-native predators such as rats and cats are introduced to islands. Lacking innate caution, even the largest seabirds can become the defenseless prey of predators as small as a mouse.
Developing a taste for meat
During World War II, the Kuaihelani Islands were cleared and covered with war infrastructure. Both black rats and house mice were inadvertently introduced at this time. Soon the rats began decimating populations of burrowing seabirds.
When Kuaihelani’s military importance faded 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 persisted. They thought they were small and harmless, but didn’t cause much concern until 2015.
While scientists may never know exactly why mice started attacking and killing mōlī, we have some ideas.
Due to climate change, Kuaihelani has experienced increasingly erratic rainfall, sometimes resulting in long dry spells or intense rain showers. During dry periods, vegetation dies quickly. It is likely that the usual food items for mice, namely seeds and insects, decrease during these periods. To survive, mice must find another food source.
On an island with millions of birds, there are plenty of seabird carcasses that attract a rich community of insects including cockroaches, woodlice and maggots. Mice seem to have quite an appetite for these creatures and probably feed on seabird carcasses at the same time. The transition from scavenging dead seabirds to attacking live seabirds that don’t fight back is just a small step.
When mouse attacks on nesting mōlī escalated from 2015 onwards, it was clear something had to be done – and quickly. The solution was to get rid of the mice, which unfortunately is much easier said than done.
Die-hard mice
Eradicating mice is a challenging 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 mice are hungriest and most 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 organisms’ unique chemical fingerprints – my colleagues and I were able to figure out which organisms mice ate and in what quantities. We found that mice on Sand Island Kuaihelani mainly eat insects (about 62% of their diet), followed by plants (27%) and finally albatrosses (probably mōlī, about 12%). The Fish and Wildlife Service has identified July as the best time for the eradication effort, as seabird densities are typically 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 rigorously 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 failed.
Some conservationists believe eradication should be attempted again, but others worry about creating rodenticide-resistant mice. When generations of rodents are repeatedly exposed to rodenticides, they can begin to carry genetic mutations, resulting in resistance to the poison, making future eradication efforts ineffective.
There is no doubt that mice on Kuaihelani have been exposed to rodenticides for a long time. When Kuaihelani – or Midway Atoll – was a naval base, rodenticide was likely applied in and around buildings and homes. The eradication of rats in 1996 was another risk. 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, especially in Europe, there are increasing cases of rodents carrying resistance. Rodents continue to have serious and widespread ecological impacts on islands around the world.
For now, I’m focusing on helping the mōlī of Kuaihelani survive. But our research can also contribute to the growing challenge of drug-resistant mice around the world.
This article is republished from The Conversation, an independent nonprofit organization providing facts and analysis to help you understand our complex world.
It was written by: Wieteke Holthuijzen, University of Tennessee.
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Wieteke Holthuijzen 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 U.S. Fish and Wildlife Service and Island Conservation.