Comparative Ecology of Pinyon Mice and Deer Mice in Mesa Verde National Park Part 6

In Mesa Verde the deer mouse, _P. maniculatus_, prefers open areas having dense stands of gra.s.ses, or brushy areas adjoining open terrain.

This species lives in stands of big sagebrush; in gra.s.sy areas having an oak-chaparral or mixed-shrub-overstory; and in gra.s.slands without shrubs, such as on the southern end of Moccasin Mesa. Pure stands of sagebrush did not support large numbers of mice unless there was additional cover nearby in the form of shrubs or oak brush.

Optimum habitats for _P. maniculatus_ were on Navajo Hill, in the burned areas on Morfield Ridge, on the northern end of Wetherill Mesa, and in the gra.s.sy areas near the entrance of the park. The trapping areas in the first three mentioned had heavy growths of gra.s.s and an overstory of shrubs.

Some individuals of _P. maniculatus_ ventured into pinyon-juniper woodland and entered traps. Such animals usually were found in places having a heavy understory of sagebrush, or in disturbed places within the woodland.

_P. maniculatus_, but not _P. truei_, was taken in the arid pinyon-juniper-bitterbrush stand on the southern end of Wetherill Mesa.

_P. maniculatus_ also was present, in about equal numbers with _P.

truei_, in a pinyon-juniper-muttongra.s.s stand north of Long House. Both of these localities supported only a few mice.

_P. maniculatus_ is found more frequently in pinyon-juniper woodland when the population density is high, and when such woodlands adjoin gra.s.slands or sagebrush areas. As mentioned earlier, _P. truei_ and _P.

maniculatus_ occur together in ecotonal areas between the forest and gra.s.sy or brushy areas. In Mesa Verde the deer mouse inhabits exposed gra.s.sy areas that have mostly shrubs in the open canopy.

_P. maniculatus_ is the first to colonize areas that have been burned; this species invades such areas as soon as primary successional vegetation becomes established. It can be stated that in general, _P.

maniculatus_ will be found in the harsher, more arid habitats. If the habitat is so inhospitable that only a few mice can survive there, _P.

maniculatus_ will be present. _P. truei_ apparently requires the more moderate conditions found in the pinyon-juniper forest, and this species does not venture far from the edge of the forest.

NESTING AND NEST CONSTRUCTION

Ten individuals of _P. truei_ and three of _P. maniculatus_ were followed to their nesting places. Photographs were taken of the nesting sites before and after uncovering. Plants or other materials used in their construction and any commensal arthropods present were saved and later identified.

Nests of _P. truei_ usually were a.s.sociated with juniper trees. Dead branches and trunks of juniper trees decay from the inside, and the resulting hollows are favored sites for the nests. Pinyon pine trees tend to decay from the outside and were not used as nesting sites by _P.

truei_. Nests of _P. truei_ were found in hollow trunks and branches of otherwise healthy juniper trees, and in hollow logs lying on the ground.

The heartwood apparently rots rapidly in juniper trees, but the sapwood remains intact for many years--even after the tree is lying on the ground. For example, a part of the pinyon-juniper woodland on the southern end of Chapin Mesa was burned in 1858, and the hollow trunks of junipers were still standing in 1966. Almost all of the pinyon pine trees that were killed by that fire have since decayed; their former presence is verified only by the crumbling remnants of their trunks that lie on the ground throughout the burned area.

The following accounts ill.u.s.trate the preferences of the two species of mice in selection of nesting sites:

No. 105, _P. truei_, adult. On July 22, 1964, after being released from a trap, this female ran to a serviceberry bush 10 feet south of station I4d, preened herself, ate a berry from the bush, and disappeared under a large rock at the base of the bush. Subsequent excavation revealed a large nest composed of gra.s.ses (_Poa fendleriana_, _Sitanion hystrix_, _Agropyron smithii_, _Koeleria cristata_), and a few leaves of serviceberry. There were three entrances to the nest, one on each side of the rock.

This mouse was captured again on August 12, 1964, released and followed to a hollow juniper log 15 feet south of station C7b, and 245 feet from the above nest. This log was dismantled, but no nest was found. A large number of chewed juniper seeds around the log indicated that this mouse, or others, had frequented the area.

On August 20, 1964, this female was followed to a large juniper log 20 feet northeast of station I4b. A small nest of shredded juniper bark was found inside the log, and there were numerous nuts of pinyon pine and seeds of Utah juniper that had been gnawed open. This site was about 320 feet from that at C7b, and about 240 feet from station I4d (Fig. 15).

No. 118, _P. truei_, young. On August 29, 1963, this male ran into a hollow branch of a partly dead juniper tree 15 feet south of station C5d. Part of this branch had been sawed off at some earlier time, and a hole about one-and-a-half inches in diameter was present in the center of the remaining part. The branch was not dissected, but probing revealed that the hole extended far into the branch and enlarged as it approached the trunk.

No. 177, _P. truei_, adult. This lactating female ran into the hollow trunk of a juniper 10 feet north of station G7a. Both lateral branches of the main trunk were rotten and hollow, but the tree appeared to be healthy. Chewed juniper seeds were present in the trunks and around the base of the tree.

This female later ran to a juniper log 30 feet north of station N4d.

Apparently there was no permanent nest at this site (Fig. 15).

No. 178, _P. truei_, adult. This female ran into a hollow juniper tree 10 feet south of station H3c. Hundreds of old juniper seeds, with their embryos chewed out, were present at the base of the tree. The tree was not cut down.

No. 238, _P. truei_, adult. This male ran into a dead juniper log 10 feet south of station O4b. Chewed juniper seeds were present on the ground, but no nest was found in the log.

[Ill.u.s.tration: FIG. 15: Diagrams showing estimated home ranges of six individuals of two species of _Peromyscus_, and location of these ranges in the trapping grid. Nesting or hiding places are described in the text, and are indicated on each diagram by an X. Shaded areas represent home ranges estimated from trapping records for 1963; outlined, unshaded areas represent estimated home ranges for 1964.]

No. 241, _P. truei_, adult. This male ran into a small hole at the base of a juniper tree 25 feet south of station G7c. The hole was at the fork of the tree, four inches above the ground, and led to a large subterranean chamber in the basal part of the trunk.

This male later ran into a dead juniper log lying on the ground 20 feet southwest of station N3b. No nest was found in the log.

After another capture, this mouse ran to a small juniper log 40 feet southeast of station G3d. There was a nest of shredded juniper bark and many juniper seeds inside the log (Figs. 15-17).

No. 245, _P. truei_, adult. This female ran into a large, hollow juniper log 20 feet northwest of station D4d. No nest was seen, but chewed juniper seeds were noted in and around the log (Fig. 15).

No. 251, _P. truei_, juvenile. This female ran into a dead juniper log beside station P4b. Chewed cones of pinyon pine and chewed juniper seeds were on the ground. A small nest of shredded juniper bark, and a few leaves of serviceberry, were found inside the log. Chewed pinyon nuts and juniper seeds also were present in the nest.

[Ill.u.s.tration: FIG. 16: (above) Photograph of juniper log at station G3d, which contained the nest of _P. truei_ # 241.]

[Ill.u.s.tration: FIG. 17: (below) Photograph of dissected juniper log at station G3d, showing the nest of _P. truei_ # 241, at end of mattock handle. The nest of shredded juniper bark contained chewed seeds of juniper trees.]

No. 267, _P. truei_, juvenile. This male ran into a fallen juniper log 40 feet southwest of station P7a and then disappeared into a hole leading under an adjacent rock. Dissection of the log revealed many chewed juniper seeds inside and beneath the log, but no nest. I did not overturn the large rock or excavate under it.

No. 268, _P. truei_, adult. This pregnant and lactating female ran into a hollow branch of a partly-dead juniper tree 10 feet south of station O7d. The limb and base of the tree were hollow, and there were large numbers of chewed juniper seeds nearby. Because of time limitations, the branch was not dissected.

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 (_Th.o.m.omys 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 gra.s.ses (_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 a.s.sociated 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, gra.s.ses 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 gra.s.ses and a few leaves.

It seems unlikely that compet.i.tion for nesting sites between the two species of _Peromyscus_ affects the local distribution of each species.

The a.n.a.lysis 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 ma.s.s 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 compet.i.tion 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 spa.r.s.e in the trapping grid, and the habitat these mice occupied was such as to make following them extremely difficult.

In captivity, both species constructed nests that were indistinguishable to me, when the mice were given cotton, kapok, or pieces of burlap as building material. The cotton or kapok was used directly, but the burlap was shredded into a fine ma.s.s of fluffy fibers. The burlap seemed to me to be the best building material, for it maintained its shape best.

Both species constructed nests that resembled inverted bowls. Solitary mice naturally built smaller nests than those built by females with young.

The entrance to the closed nests varied; often the female would bolt through the side of the nest where there was no opening. Sometimes the mice would exit and enter through the top of the nest. In some cases it appeared that the entire nest was closed; probably the occupant had closed the entrance. Such a closed nest would have the advantage of greatly moderating the microenvironment within the nest, and would allow the animal within to remain comfortable with a minimum expenditure of energy. The larger nests found in the trapping grid resembled those built by captives. Nests built of gra.s.ses were always larger than those built of juniper bark. Juniper bark is as easily worked into nests as are gra.s.ses, in my judgment. Therefore, difficulty of construction of nests from this material probably does not account for the smaller size of the nests composed of bark. I think the difference in insulating characteristics between the two materials probably accounts for the difference in size of the nests.