We get a lot of questions about birdhouses in March as we head into the nesting season and people wonder about any maintenance or things they should do ahead of the coming spring. A new question came in this week from Stephen Forrest of Brunswick: “Something has been chipping away at the entrance to my bluebird nesting box, almost as if (it) is trying to widen the opening. What is going on?” In the photo he provided, it looks like someone or thing has been chiseling away at the wood an inch or two all around the circular opening.

Birdhouses are just a replacement for a natural cavity that occurs in the landscape. These can be formed when limbs break off trees, or can be created by primary cavity nesters, like woodpeckers. Primary cavity nesters excavate their own holes for nesting but will only use it for one season, and then leave it to be used by secondary cavity nesters. A fairly diverse group of birds will use cavities, but other animals, particularly mammals, are very fond of cavities and all the protections they provide. Let’s look at those and see if they may be the culprits behind this behavior.

We have several species of mice in Maine, and different species use different techniques for making it through the winter. Maine’s two species of jumping mice are true hibernators. They’ll go into a state of significantly lowered metabolism, including lowering the heart rate, body temperature, and respiratory rate. Then we have our deer mouse and white-footed mouse, which will stay active in the winter. In order to stay active, these mice need to have lots of food available to them and will often take over cavities, especially birdhouses, as a place to cache their food. Some will even gather insulating materials for bedding. Mice are small enough that they can usually enter a cavity with ease, which eliminates them as suspects from Stephen’s mystery.

As we go up in size on the mammal scale, we start to encounter other likely candidates. Eastern chipmunks typically prefer using underground burrows for most of their nesting and food storage. However, if we look at a red squirrel, only a couple of inches larger than a chipmunk, we find a mammal that I think of as being more likely to expand an opening on a birdhouse. While we might see this as destructive, it is hard to blame them – it’s exactly what they would be doing with any natural cavity as well.

I’ve also seen flying squirrels do this. A cavity made by a hairy woodpecker on the edge of Maine Audubon’s parking lot was slowly widened throughout the winter and finally one day I saw a large-eyed flying squirrel peeking out at me. We are also quickly entering the gray squirrel nesting season and they’ll be building lots of nests and dreys in trees, but also looking for cavities they can take over.

If you have birdhouses up and are worried about mammals moving in, there are a few types of guards you can use. Smaller mammals, like the mice mentioned above, are tough to deter but cleaning out any material they’ve amassed through the winter will often be enough to encourage them to move on. There are metal brackets, often called “hole restrictors” or “hole guards” – they apparently also go by the slightly sinister “squirrel tooth benders” – that have a drilled opening the same size as the intended entrance to the cavity. An easier homemade option: make your own guard by adding an additional board around the birdhouse entrance, with a hole the same size as the one in the birdhouse. Just adding this extra half inch or more of material that a predator would need to gnaw through will likely be enough to deter them.

The nesting season, for both birds and mammals, is ahead of us. This is a good time to get out and give your birdhouses a quick cleaning, and remove any old material that may be decaying or harboring parasites. Make sure the houses have good drainage. Or consider hanging a new one.

Do you have a nature question for Doug? Email questions to ask@maineaudubon.org and visit www.maineaudubon.org to learn more about birding, native plants, and programs and events focusing on Maine wildlife and habitat. Doug and other naturalists lead free bird walks on Thursday mornings, 8 to 10 a.m., at the Gilsland Farm Audubon Sanctuary in Falmouth.

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