We have more than 100 nestboxes on our property, which I monitor weekly during the avian breeding season. These nestboxes provided me my first acquaintance with the 2 deer mouse species we have in our area–the Pinyon Mouse and the Deer Mouse. Both of these species are more likely to build nests on the ground—in rock piles or under bushes, for example. But they both build nests in the boxes and raise young in our nestboxes too. But these nests are quite different structurally and the behavior of the mice in the boxes when you open the box is dramatically different.
A Deer Mouse nest is typically very deep, made up of very fine, thin strips of juniper, built almost to the bottom of the entrance hole of the box. They typically have a hole that provides almost a tunnel for access to the center of the nest either on the top or, occasionally, on the side of the nest. Chipmunks also nest in our boxes; their nests are similar in structure to a Deer Mouse nest. But they use larger strips of juniper bark and their nests tend to have a more open, bowl-shaped nesting area toward the top of the nest. (You can see photos of a Deer Mouse nest and a chipmunk nest here; click on any of the photos there to see a larger version.) A Pinyon Mouse nest (right) is much shallower than a Deer Mouse nest, with far fewer strips of juniper and more often dried vegetation as the base of the nest. If I see oak leaves, I know it can only be a Pinyon Mouse nest. They also seem to like to incorporate animal fur—sometimes even the fur from our dogs, which we put out in an old suet cage during breeding season—and occasionally even feathers.
In June, 2011, I came across a Pinyon Mouse actually nursing 2 young (below)—much to both of our surprises. (Click on the photo to see a larger version.)
Finally, one fall day when I was doing the final clean-out of the nestboxes in preparation for the winter, I came across a pair of Pinyon Mice—both seeming to be adults—in a nestbox. You can see photos of this duo here; scroll down to the bottom of the page.)