The nest boxes on our property are all hung on trees. Note that we don’t have many of the predators that other areas do. (You can read more about that here; click on the “mounting of the boxes” link in the first paragraph.) In areas with many predators, the recommended practice is to mount nest boxes on poles with appropriate predator guards. (You can read more about those practices here.)
Because our boxes are on trees, two species of small mammals nest in them as well: the deer mouse and the least chipmunk. (Well, I think they are least chipmunks. The books suggest that we may also have slightly larger and very similar Colorado chipmunks in our area. I’ve been trying to catch sight of the telltale differences: e.g., leasts allegedly move their tails up and down when vocalizing (vs. from side to side in Colorados); the stripes on the back of a least apparently don’t reach all the way to the tail (while the stripes do reach the tail in a Colorado). So far, nothing definitive. But since least chipmunks are the most common chipmunk in the area, I’m going with that until further notice.) Not everyone would welcome these species in their nest boxes. I enjoy chipmunks, for the most part, although they can wreak havoc on wiring and many other aspects of human domesticity. And their perpetual squeaking drives the dogs crazy (although, since they are cattle dogs, that’s not a very far trip). I’m not especially fond of deer mice (especially in an area with hanta virus). But even though I don’t particularly love either of these species, I do appreciate the many species that eat these species. Also, since we have more than 120 boxes, the avian cavity nesters always have plenty of choices even if the mammals take a few boxes. So we allow chipmunks and deer mice to use our nest boxes relatively unmolested.
In our area, we seem to have a small number beginning to nest in late winter/early spring; a larger number nest in late summer/early fall. The early nesters are often building nests several weeks before even our earliest avian cavity nesters—the White-breasted Nuthatch—have started picking out boxes.
Deer mouse nests are usually relatively easy to identify. They are made primarily of very finely shredded bark, piled high with a small tunnel in the center of the top. To the left is a photo of a mouse nest in the box. (Click on any photo to see a larger version.) The bark shreds are packed into the box, filling it from corner to corner. When it is first constructed, the nest is also usually quite deep—reaching from the floor almost to the bottom of the entrance hole. It gets tamped down through use, though. So by the end of the season, it’s probably only half the height from the floor to the entrance hole. (You can see a schematic of our typical NABS-style box here; click on the “overview” link in the first paragraph.) To the right is a photo of a mouse nest that I had removed in the end-of-season cleaning, where you can really get a sense of the entrance “tunnel.” (Since the nests are packed into the box so tightly, they usually come out as a single piece.)
Chipmunk nests are typically made of larger strips of juniper bark, with some leaves and other sundry vegetation on occasion. They are much more loosely built than mouse nests. And since they begin as strips of juniper, I often cannot tell whether the beginnings of a nest are chipmunk or Mountain Bluebird (which also uses predominantly juniper bark here). However, a chipmunk nest remains rather a jumble of strips while a MOBL nest evolves into a sculptural, swirly object of beauty. Chipmunk nest depths vary—some reach nearly to the entrance hole (such as the one in the photo on the left) and some reach less than half the height of the box (such as the one in the photo on the right). Occasionally, a chipmunk will build a nest on a nest that a mouse began, just to keep you on your toes. When that happens, you can see the layers of finer shreds of juniper in the foundation, with larger strips of juniper added a bit more haphazardly on top.
I spotted the chipmunk below when I was checking boxes in mid-April. Chipmunks are usually quite skittish, ducking or running when they see any movement. This one watched Paxi and me from the entrance hole, not budging from this position for at least 2 minutes—all while I was dropping my monitoring bag, fumbling for (and then with) the camera, and snapping several shots. To me, she looked as if her kids were driving her crazy, she was bored silly, and she was longing for the good ol’ single life. Hey, a little anthropomorphism doesn’t hurt every now and then…