Of Mice and Mōlī

By Hob Osterlund, Safina Center Fellow

During the annual albatross count at Midway Atoll in December of 2015, volunteers reported seeing nesting albatross with bloody heads and necks. The wounds left biologists scratching their own heads.

“We started trying to imagine what kinds of predators might have made their way to the colony,” said Beth Flint, Wildlife Biologist for the Marine National Monuments of the Pacific.  “At first we thought it might be a migratory raptor like an owl or a hawk.”

The team got a definitive answer—and a real surprise— from automatic trail cameras set up near the injured birds. 

The predator was a mouse.

Non-native mice (Mus musclaris.) and black rats (Rattus rattus) arrived on Sand Island—the largest of three islands of the atoll— during military occupancy more than seventy-five years ago.  Rats were eradicated in 1996, so mice became the only non-native mammal in the entire Northwestern Hawaiian Islands.

Midway Atoll National Wildlife Refuge is approximately twelve hundred miles northwest of Honolulu, and is a part of the Papahānaumokuākea Marine National Monument (PMNM.) Every year an estimated three milllion birds from more than two dozen species take advantage of Midwayʻs flat open spaces for resting, courting, nesting and raising their young. It’s home to the largest colony of albatross in the world. 

Laysan and black-footed albatrosses court and nest on Midway Atoll. Photo: Hob Osterlund

The islands are crucial breeding grounds for both Laysan (Phoebastria immutabilis)— also known as mōlī in Hawaiian— and black-footed albatross (Phoebastria nigripes.) Since 2015, mice attacks have grown from a few isolated incidents in confined areas to hundreds of events covering a much larger portion of the refuge.  The attacks not only cause injury, they can also lead to egg abandonment and adult death. 

In the 2015-2016 hatch season, mouse attacks occurred at three sites on the island. Map: US Fish and Wildlife Service Pacific Region.

During the 2016-2017 hatch season, the attacks spread across the entire island. Map: US Fish and Wildlife Service Pacific Region.

Mouse predation is the stuff of horror movies, and was very distressing for the team to witness. “The wounds were what you would expect if one animal was chewing on another,” said Flint.  The bites broke the skin and often went into muscle and fat, and even sometimes into body cavities.

Volunteers and staff also found an increased number of abandoned nests. “It looked like the birds had been attacked and then said I can’t take this, I’m outta here,” said Flint.  “That’s a significant thing for an albatross, because they are very devoted parents.”

Biologists have not yet determined what caused the mice to morph into albatross predators. Although they are known to be omnivorous, they had not previously been considered even a remote threat. There had never been a single documented case of mice predation on adult mōlī before 2015. And whatever the trigger, there was a potential for incredible damage.

Now, in order to prevent the attacks from getting worse, the U.S. Fish and Wildlife Service has proposed a strategy to remove all mice from Midway Atoll.  A Draft Environmental Assessment for the Midway Seabird Protection Project (draft EA) is now available for review.  All alternatives, environmental effects and mitigation measures are evaluated in the draft, which you can access here: https://www.fws.gov/refuge/Midway_Atoll/ or here:  www.papahanaumokuakea.gov.

The plan describes eradication of mice from Sand Island by delivering an aerial and hand-broadcast lethal dose of Brodifacoum 25-D Conservation—a rodenticide delivered in a cereal pellet—which has been successful on dozens of islands worldwide. There are no predators in danger of secondary poisoning the Northwest Hawaiian Islands.  The dispersal will happen in the summer, when the lowest numbers of birds are present.  Careful protection of the non-migratory, critically endangered Laysan duck will be necessary.

Until then, the albatross population is well beyond the riskiest period of this season, when the adults are incubating eggs and are, as it were, sitting ducks. Fortunately, there is no evidence that the mice are predating on the chicks.