Mice Are Starting Annual Migration Indoors

Mice Are Starting Annual Migration Indoors

  • Editor’s Note: This “Good Natured” article was submitted to Kane County Connects by Pam Otto, the manager of nature programs and interpretive services at the Hickory Knolls Discovery Center, a facility of the St. Charles Park District. She can be reached at 630-513-4346 or potto@stcparks.org.

This week’s column comes in the form of a public service announcement courtesy of my cat, Jimmy.

Being a cat of few words, except when it’s supper time, Jimmy wanted you to know just two things:

  • Mice are starting to make their annual migration indoors, and
  • Poison is not the way to get rid of them.


Jimmy delivered the first part of this important message late the other night. Or maybe it was early the other morning. Either way, it was dark, I was sleeping, and Jim had to make several attempts before he got his point across.

Each time, I was awakened — though barely — by the sensation of something warm, wet and vaguely animate being flipped up, on and around the region of my head. I’d stir a bit and Jimmy would take off, running out of the room and down the hall meowing like some sort of feline town crier.

It was light out by the time I became conscious enough to fully grasp what Jim had been trying to tell me. I did so with one hand, picking up a soggy and quite dead Mus musculus that was lying — I kid you not — mere inches from where my other furry companion, Joey the dog, had slept soundly the entire night.

I couldn’t believe it. (Not the part about Joey. He’s a sound sleeper, except when there’s thunder.) But there it was, a house mouse. In my house. The first one ever to breach the perimeter in 23 ½ years.

It was time to take action. But first, a little background on the history of Rodentia at Casa Otto.

Though this recent event marked the first time a mouse was found inside the house, my attached garage has had several rodent visitors, mainly in fall and winter, over the years. They were a peaceable bunch, and as I don’t store any food or other items out there that they could damage, they were allowed to stay. I’d occasionally find evidence in the form of a seed cache, and once in a while would catch a glimpse of the chipmunk or mousy that had left it, but never found any signs of “infestation” — that term pest removal firms love to toss around.



I suppose I should also add that the garage mice have always been Peromyscus spp, probably white-footed mice, native species that are staples in the diets of our local predators. If letting them take shelter would result in a more robust hawk and owl population, I was all for it.

I did have to modify my attitude slightly last year, the first and only time I had a negative interaction with my little rodent tenants. As a gift I’d bought a birdseed mix that contained lots of nuts, including very fragrant peanuts, and was keeping it in the car until I had a chance to deliver it.

Alas, the temptation proved too much for the perceptive Peromyscus. They chewed their way into the car’s interior by way of some plastic parts near the fuel tank. The resulting spray of fuel was enough to deter them from getting into the car’s interior, while the subsequent $600 repair

Mouse in the kitchen eating bread

House mouse

bill was enough to deter me from letting the mice stay. A multi-pack of Victor snap traps and a bit of peanut butter later and the garage population of rodents dropped to zero.

With Jimmy’s discovery, however, a new era dawned. House mice — Eurasian species that need humans in order to survive — can quickly overpopulate an area. Unlike their wild cousins, which breed only when weather conditions are favorable, house mice reproduce year round. With a gestation period of 21 days and an average of seven offspring per litter, these guys can quickly populate, and overpopulate, an area.

That’s where the second part of Jimmy’s message comes into play. House mice are pests, without a doubt. But poison baits are not the solution.

Sure, they seem convenient. Neatly packaged blocks dressed up with scents touted as attractive to rodents, these products promise to kill mice and rats in just a few days. That end result would be all good if dead rodents were indeed the only outcome. The problem is, it’s not.

About Rodenticides

Many of today’s rodenticides are designed to kill pests over the course of a few days. This method at first seems counterintuitive; you’d think sooner would be better.

Here’s where a little more history is helpful.

Old-school poisons, like strychnine and arsenic, provided immediate results. They acted so fast that the rodent died instantly. Thing was, a fallen comrade lying on a heap of poison was all the warning other rodents needed to avoid the bait entirely. Instead of a problem solved, people ended up with one critter down but lots of others still doing fine, thanks to their pal who took one for the team.

dead Mouse in cheese trap over yellow background

As a consequence, many of today’s baits fall under the classification of second generation rodenticides. They contain active ingredients like brodifacoum and difethialone which are designed to act slower, sometimes requiring more than one dose and taking several days to kill the intended target. Over this period of time, the mouse or other species moves about and, in its weakened condition, can become easy prey for predators — wild ones like hawks and owls, as well as domestic ones like dogs and cats.

Sure, the outcome is the same — the rodent dies. But so too does the animal who ingested it, often in slow and agonizing fashion.

The folks at the National Audubon Society wrote at length about the phenomenon of rodenticides traveling up the food chain. It’s an excellent article that can be accessed here.

The gist of the piece, as indicated by the link, is that safer methods of mice control exist. Making the point that we needn’t use “a sledgehammer when a flyswatter will do,” the article lists several alternatives to second-generation rodenticides, including my go-to, the snap trap.

In dealing with the most recent development inside my own home, these devices will certainly play a role in the solution, as will finding and filling gaps that may have provided little M. musculus with access. (Here’s a fun fact: Mice can squeeze through any opening that is wide enough to accommodate their head, which is about as big around as a dime.) Steel wool is an effective deterrent, as is copper mesh — which offers the additional benefit of not rusting.

The majority of homes that live in areas where there are a lot of animals discover that they suffer with having to find ways of getting rid of pests in their home. If this is something you can relate to, you don’t have to worry any longer, as looking into something like a Phoenix pest control service (if you live in and around this area of Arizona) will help you claim back your property from unwanted visitors.

Note that live traps, those seemingly humane contraptions designed to capture but not kill, are not an option I’ll even consider. Why? There’s no place to release the captured critters. Putting the mouse back outside merely inconveniences the animal for a short period of time, while dropping it off in a park or forest preserve is an ordinance violation. Legalities aside, relocating a rodent sentences it to a lengthy, confusing and inhumane death — not to mention creating problems for the resident rodents it surely would encounter.

As we progress through the autumn months, mice will continue their progression too, making preparations for the winter to come. Now’s the time to educate yourself on safe and sustainable means of rodent control. Hawks, owls and other native predators will appreciate your enlightened approach. And Jimmy will, too.