Part 6 (1/2)

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.]