This is why you’re seeing more mice this time of year

mouse-in-lush-green-vegetation-forest-floor Photo by Rudmer Zwerver/Shutterstock

I drove back from the cottage tensely anticipating the snap, snap of two baited mousetraps in the back of the car. I was sure I wasn’t alone on this drive. I’d found tiny scraps of tissue scattered over the floor and a plastic baggy in the backseat had the tell-tale nibbled punctures. But at this point, I was hardly surprised.

This was the summer and fall of mice. We set traps in the cottage, closely guarded our food on camping trips, and gathered around bonfires swapping stories of rodents taking refuge in cupboards and drawers. It’s not unusual to find mice nestled into your cottage in the winter, but this seemed like a lot for the summer.

You might have heard it explained as thirsty mice looking for water and a place to cool down during a hot, dry summer, but Jeff Bowman, a wildlife research scientist with Ontario’s Ministry of Natural Resources and Forestry, says it’s all about the trees.

“What happens with mice is they really fluctuate a lot from one year to another depending on food supply, in particular in the fall,” says Bowman. “The fact that we had a lot of mice this summer of 2018 would mean there were really good tree seed crops during the fall of 2017 — maple keys and acorns, depending where in particular you’re talking about.”

Beechnuts, maple keys (or samara) and acorns from oak are all an important source of food for mice. These fruits of the trees are their mast. A bumper crop of tree fruit, or a mast event, means mice have a lot of food for the fall and can produce at a faster rate, says Bowman. They can even breed in the winter if the conditions are right. “That’s what we see when we look at our tree seed data,” he says. “Last year was a good year.”

So, what drives these mast events? It’s actually a response to a less fruitful year — literally. Something like a late frost or a drought could cause tree seeds to fail one year, so they have to make up for it the next year with a mast event. It’s an adaptive strategy to ensure the species thrives. And because weather conditions that cause a failure one year aren’t isolated to one small area or species, their impacts are quite broad. “A late frost or something like that, you can have several different tree seed species that fail and when that happens they become synchronized,” says Bowman. “And you’ll have widespread masting.”

The good news is these years of abundant food fluctuate. A massive amount of energy goes into a mast event, so the following year, trees would need to conserve. In other words, next summer shouldn’t be quite as bad. Bowman says one way to predict whether there will be an influx of mice the coming summer, is head out for a fall walk in the woods. If you see a lot of maple keys and acorns on the ground, it might be time to mouse-proof the cottage.

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