No. 74, P. maniculatus, juvenile. This female ran into a small circular hole in the ground 13 feet north of station J3a. Excavation revealed that this hole led into the abandoned tunnel of a pocket gopher (Thomomys bottae). The tunnel was followed for about four feet, but no nest was found and the tunnel led under a thicket of oak brush which made further excavation impractical ([Fig. 15]).
No. 247, P. maniculatus, adult. This male was followed to a large nest situated at the base of a stump and under a juniper log lying beside the stump, five feet from station I2c. This large nest was built on the ground and was constructed of grasses (Poa fendleriana, Stipa comata, and Koeleria cristata), and contained a few leaves of Gambel oak. It was the largest nest found. Chewed pinyon nuts were in the nest. ([Fig. 15]).
No. 276, P. maniculatus, juvenile. This male ran into a small hole at the base of a dead juniper tree 40 feet north of station O2c. It would have been necessary to cut the tree down to uncover the nest, and this was not deemed to be worthwhile.
The preceding accounts indicate that, in Mesa Verde, nests of P. truei usually are associated with hollow juniper logs or branches. In one instance a nest of P. truei was found on the ground, under a rock. Shredded juniper bark, and, in one case, grasses were the materials most commonly used for nest building.
Individuals of P. maniculatus did not build nests in trees. One nest was found under a stump and adjacent log. Another site was in the abandoned tunnel of a pocket gopher, and a third was under a large rock. The only nest that was unquestionably built by a P. maniculatus was constructed of grasses and a few leaves.
It seems unlikely that competition for nesting sites between the two species of Peromyscus affects the local distribution of each species. The analysis of nesting sites suggests that P. truei is restricted, in Mesa Verde, by the availability of fallen logs, hollow branches, or hollow trunks of juniper trees. My observations lead me to think that within the pinyon-juniper woodland there is a surplus of nesting sites for individuals of P. truei. Many juniper trees have dead branches, and hollow juniper logs are abundant throughout the forest. It is inconceivable to me that the population of P. truei could reach densities sufficient to saturate every nesting site available to them in the trapping grid.
Sagebrush areas, or brushy zones adjacent to the pinyon-juniper woodland usually do not contain juniper logs; when hollow juniper trees or logs are not available, P. truei is not found as resident of such areas. As mentioned earlier, individuals of P. truei may venture into such areas to feed if they are adjacent to pinyon-juniper woodland.
An individual of P. truei may have more than one nest within its home range (for example Nos. [105] and [241] cited above). Each mouse probably has refuges, each containing a nest, strategically located in its home range. Thus, if a mouse is chased by a predator, or by another mouse, it need not return to its main nest, but can seek refuge in one of its secondary nests. These secondary nests were small and were invariably constructed from shredded juniper bark. Some of these nests were little more than a scant handful of shredded bark that formed a platform to sit upon. Other nests were larger and ball-shaped, with one opening on the side. All of the secondary nests that were found were inside hollow juniper logs. The bark used in construction of the nests had, in each case, been transported from nearby living trees. The logs had previously lost their bark through decay.
The evidence indicates that these secondary refuges are prepared with considerable care. Not only is the bark transported for some distance, but it is shredded into a soft mass of fibers. When a mouse first establishes itself in a new area, perhaps it begins several such nests before settling upon the most favorable site. The less desirable sites, if still within the animal's range, are then available (barring competition by a new inhabitant) for outlying refuges.
My data do not indicate whether individuals of P. maniculatus use a similar arrangement of nests within their home ranges. The population of P. maniculatus was sparse in the trapping grid, and the habitat these mice occupied was such as to make following them extremely difficult.