An adult owl brings a mouse back to a nesting box set up in an orchard in Hirosaki, Aomori Prefecture. (Provided by the Faculty of Agriculture and Life Science, Hirosaki University)

HIROSAKI, Aomori Prefecture--Farmers in Japan’s main apple-producing region are encouraging the wild owls they once turned off back to their orchards as saviors in their battle against tree-killing rodents.

The apple-growing community here in the northernmost prefecture of Japan’s main island, where 60 percent of apples are produced, found that replacing the tall trees with shorter farmer-friendly ones meant that owls fled and mice proliferated.

Now, the pest-killing birds are making a comeback.

In an approximate 2,000-square-meter orchard in Hirosaki's Shimoyuguchi district, a nesting box supported by 2-meter legs stands a head taller than the apple trees surrounding it.

It is currently vacant, but apple grower Chikage Ishioka, 35, said, “Owls will come back in the spring to raise chicks.”

In 2014, Ishioka founded Shimoyuguchi Fukuro no Kai (Shimoyuguchi owl association) with fellow farmers in the neighborhood. The group has set up dozens of nesting boxes for owls in orchards in cooperation with a research team led by Nobuyuki Azuma, professor of animal ecology at Hirosaki University’s Faculty of Agriculture and Life Science.

In past times, hollows of large apple trees used to provide ideal nesting spaces to lay eggs and rear owlets with a good supply of wild mice. However, tall trees with hollows have been replaced by smaller species to optimize production since the 1970s, and owls disappeared from orchards, leaving humans to fight the rodents alone.

It is said an apple sapling requires seven to eight years to become profitable. In winter, mice feed on apple wood when there is nothing else to eat. In some cases, the damage goes unnoticed until spring, as snow accumulates up to 1.5 meters in the area, and young trees sometimes die as a result.

As the Agricultural Chemicals Control Law restricts the number of times rat poison can be used, apple farmers are resorting to spending large amounts of time and money on mice control measures, including setting up traps and mice guards on trees, as well as mowing grass in orchard grounds.

For owls that used to inhabit apple orchards, mice were an essential part of their diet while nurturing their chicks. The owls usually spent the rest of the year on trees in forests or bushes, but they would fly in to the human habitat to lay their eggs in hollows of apple trees around March. For about two months after hatching and until the owlets left their nests, parents would feed mice in the orchards to their young.

Owls’ role in controlling mice was not recognized much until recently.

However, when a Hirosaki University team conducted research in 2016 on the mice population around the nesting boxes Ishioka’s group set up, the results were astounding.

Compared with April, prior to the incubation of owlets, the mice population in the 100-meter radius of occupied nesting boxes dropped by 70 percent on average in May, excluding expected natural attrition.

In another survey, the number of mice an owl caught in a day was almost equal to what a mouse trap usually snares in a week.

“I never imagined they were doing such an amazing job,” said Ishioka.

Seeing the tangible effect of owls in Hirosaki, apple farmers in Aomori city and Nagano Prefecture also set up nesting boxes in their orchards in 2016. Ishioka continues to receive viewing requests and inquiries about his project.

“Having owls in orchards can reduce (mice control) costs and labor, and that is a great help for farmers who are facing a fast-aging workforce,” said Ishioka.