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The eradication of house mice that prey on native birds on Midway Atoll has been delayed for another year due to supply chains slowed by the pandemic.

U.S. Fish and Wildlife has been planning an eradication program on Midway Atoll National Wildlife Refuge since 2015, after it discovered that, despite the mismatch in size, house mice were climbing onto and biting Laysan albatrosses, wounding and sometimes killing them.

The federal wildlife agency scheduled its first eradication program for the summer of 2019 but delayed it for years, most recently because of the pandemic. And now supply chain issues have delayed the eradication project until 2023.

Midway is a sanctuary for millions of seabirds and shorebirds vulnerable to invasive rodents, which were introduced during the World War II era when Midway was a crucial staging area for the U.S. Pacific war effort. Going westward, it is the second-to-last atoll in Papahanaumokuakea Marine National Monument’s island chain, lying more than 1,300 miles northwest of Honolulu.

Invasive animals have contributed to precipitous population declines for some species, such as , which were killed off by black rats until the rats were eradicated in 1995.

Mice were considered benign until 2015, when it was discovered they were attacking nesting Laysan albatross — 70% of the world’s population of the species breed in the refuge.

Control Efforts

Since house mice came into the sights of Fish and Wildlife, research has been conducted on how to best eradicate them. One exercise, which informed the current plan, used non-toxic bait drops to assess where and how to best distribute rodenticide across Sandy Island, the only one of three islands in the atoll affected by mice.

Current plans calls for 10 40-foot containers of bait to be used on Midway, distributed both in precision helicopter drops and by hand, according to Papahanaumokuakea monument superintendent Jared Underwood.

Fish and Wildlife’s original plan to eradicate the population in the summer of 2019 was delayed. Then the pandemic came along, Underwood says.

The problem now is getting large volumes of the required bait, which is made up of several ingredients to entice the mice to eat it and laced with small amounts of poison. It also takes time to prepare the bait, he said.

Officials also must take care to avoid collateral damage to other important species on the island. Fish and Wildlife staff plan to remove all endangered Laysan ducks from Sandy Island and return them once the bait has dissipated.

Potential harm to marine life from rodenticides was a concern in previous eradication efforts, such as killing off rats on the island of Lehua in 2017.

But Underwood said the bait is not pure rat poison.

“A pellet is not mostly rodenticide … it’s been infused with rodenticide,” Underwood said. “In reality, the amount of rodenticide we are putting on there is a small bucket.”

In the meantime, resident atoll biologists have been laying bait in specific areas to control the mice before Fish and Wildlife totally erases the population.

Audacious Mice

Mice have been known to feed on chicks of several species of birds, as they do on Midway, but feeding on adult birds such as Laysan albatrosses — whose wings can span 6 feet — is relatively novel, according to Aaron Shiels, a U.S. Department of Agriculture research biologist specializing in invasive species.

Shiels, who studied rodents as part of his Ph.D. at the University of Hawaii Manoa, says mice are opportunistic eaters but their diets are typically heavy on invertebrates and plants.

The biologist believes there are four probable factors behind the mice taking on animals higher up the food chain: a dense mouse population, a reduced food supply, a large population of birds that does not consider mice a threat and the mice learning that albatross are a viable protein source.

Nonetheless, mice feeding on large sea birds remains an anomaly.

“I know of only a few islands around the world where they have been reported eating birds, and Midway Island … is one of those oddities,” Shiels said in an email.

Seasonal Eating Animals

The behavior of the Midway mice is not uniform: Only certain groups are eating the albatross on certain parts of the island.

Another factor could be efforts to control invasive plants.

“Those weedy plants may have been a food source for the mice and probably harbored a host of invertebrates the mice relied upon for food,” Shiels added.

But in , biologists found the mice were not necessarily starving before they started attacking albatrosses.

Whatever the causes, mice pose a long-term potential threat to the global population, according to a .

But biologists say one more year will not likely lead to a population boom in mice or an extinction of birds.

The study’s authors hypothesize that the rodents’ eating habits were due to an increase in populations after the eradication of black rats and “a dietary shift prompted by as-yet-unknown causes.”

Thierry Work, one of the study’s authors and a project leader for U.S. Geological Survey, believes part of the dietary shift could have been the eradication of an invasive plant called verbisina, also known as golden-crown beard.

The plant’s seeds could have been a good source of protein, and since they have been removed from the atoll, the mice found an alternative in albatross.

“We really don’t know why they shifted to eating albatross,” Work said. “I suspect they are looking for a source of protein.”

Mice are the last invasive mammal residing in the monument, however, so eradicating them will be a significant step toward ensuring Papahanaumokuakea’s wildlife prospers, according to superintendent Underwood.

This is particularly good news for the Laysan albatross, which relies heavily on Midway.

“If something were to happen to Midway Atoll, that would be devastating to the albatross species,” Underwood said.