Part 10

Air temperatures and relative humidities at this station did not differ appreciably from mean temperatures and humidities at the other stations. The unusual feature is the lack of evidence of cold air drainage. The lake of cold air in the bottom of the drainage apparently is too shallow to reach this station. This site is near the head of the drainage, and the cold, nocturnal air probably moves rapidly down slope into the deeper parts of the canyon, rather than piling up at the shallow head of the drainage.

In spite of the shade afforded the ground by the oak brush, temperatures reached the same maximum values as at the drainage site, owing to the orientation of the slope. South-facing slopes receive more direct insolation throughout the day and throughout the year than north-facing slopes and mesa tops (Geiger, 1965:374). In Mesa Verde, south-facing slopes tend to be more arid; snow melts rapidly, and most of this moisture evaporates. As a consequence, south-facing slopes have less soil moisture and more widely-distributed vegetation than north-facing slopes where snows often persist all winter and melt in spring. (For a detailed discussion of climates on northeast-versus-southwest-facing slopes in Mesa Verde, see Erdman, Douglas, and Marr, in press.)

Pinyon-Juniper-Muttongrass Site, 7,600 feet elevation

The station was in the trapping grid at D5b (Fig. 13). The pinyon-juniper woodland surrounding this site resembles much of the woodland on the middle part of the mesa. The forest floor is well shaded by the coniferous canopy, and muttongrass is the dominant plant in the ground cover. P. truei lives in this habitat.

The climate at this site is moderate. Shade from the canopy greatly moderates the maximum air temperatures during the day; minimum air temperatures, however, are about the same as at the other stations (Table 4). Mean temperatures are somewhat lower at this site than at the others because of the lower maximum temperatures. Relative humidities do not differ markedly from those at other stations.

Figure 14 shows hygrothermograph traces at all stations for a typical week. An interesting phenomenon is illustrated by several of these traces. By about midnight, air temperatures have cooled to within a few degrees of their nightly low. At this time, heat is given up by the surface of the ground in sufficient quantities to elevate the air temperature at ground level. This release of reradiated energy lasts from one to several hours, then air temperatures drop to the nightly low just before sunrise. A depression in the percentage of relative humidity accompanies this surge of warmer air. On some nights winds apparently disturb, or mix, the layers of air at ground level. On such nights the reradiation of energy is not apparent in the traces of the thermographs. Reradiation of energy is restricted to ground level, and traces of hygrothermographs in standard Weather Bureau shelters, approximately four feet above the ground surface, at other sites on the mesa top did not record it.

The instruments used in this study were unmodified Belfort hygrothermographs containing as sensing units a hair element for relative humidity and a Bourdon tube for air temperatures. The hair element, especially, does not register changes in humidity at precisely ground level; rather, it reflects changes in the layer of air from about ground level to about a foot above. Thus data from these instruments give only approximations of the conditions under which mice live while they are on the ground.

Climatic conditions greatly influence trapping success. Larger numbers of mice generally were caught on nights when humidities were higher than average. Rain in part of the evening almost invariably resulted in more mice of each species being caught. This was probably due to increased metabolism, by the mice, to keep warm. Apparently the mice began foraging as soon as the rains subsided; mice were always dry when caught after a rain. Few mice were caught if rains continued throughout the night and into the daylight hours.

TABLE 4--Monthly Averages of Daily Means for Maximum, Minimum, and Mean Air Temperatures and Relative Humidities at Four Sites in Mesa Verde National Park, Colorado.

===================+========================+======================== Site | Maximum Temps. | Maximum R. H. | J J A S O | J J A S O | | Far View Sagebrush | 89 91 86 77 74 | 68 84 82 88 71 Drainage | 86 91 85 78 78 | 87 94 93 96 84 Oak Brush | 86 88 82 76 81 | 57 78 80 80 66 Pinyon-Juniper-Poa | 75 80 74 66 64 | 59 83 82 88 58 | | | Minimum Temps. | Minimum R. H. | J J A S O | J J A S O | | Far View Sagebrush | 42 53 50 42 31 | 18 24 25 29 21 Drainage | 36 48 45 38 26 | 21 26 27 29 30 Oak Brush | 42 52 50 42 32 | 19 25 30 31 21 Pinyon-Juniper-Poa | 44 54 50 42 34 | 22 30 29 32 25 | | | Mean Temps. | Mean R. H. | J J A S O | J J A S O | | Far View Sagebrush | 66 72 68 60 52 | 43 54 54 48 46 Drainage | 61 70 65 58 52 | 54 60 60 62 52 Oak Brush | 64 70 66 59 56 | 38 51 55 56 44 Pinyon-Juniper-Poa | 60 67 62 54 49 | 41 56 55 60 42 -------------------+------------------------+------------------------

Nights of high trapping success usually were associated with days having solar insolation below the average. Insolation was measured with a recording pyrheliometer at a regional weather station (M-2) on the middle of Chapin Mesa, at an elevation of 7,150 feet (Erdman, Douglas, and Marr, in press). This station was approximately one mile south of the trapping grid; isolation at this site would have been essentially the same as that received by the trapping grid. Below-average isolation for one day indicates cloudy conditions, which are accompanied by increased humidity, but may or may not be accompanied by precipitation. Trapping on nights preceded and followed by days of average or above average isolation with average humidities--indicative of clear days and clear moonlit nights--did not yield appreciably higher catches of mice than other nights. Hence there was no evidence that mice tended to avoid, or to seek out, traps on clear moonlit nights.

On cold, humid nights in autumn numerous mice caught in Sherman live traps succumbed from exposure, even though nesting material (kapok or cotton) and food were in the traps. Occasionally mice succumbed to heat when traps were inadvertently exposed to too much sunlight. Apparently little heat is required to kill individuals of either species. Traps in which animals died due to excessive heat usually were not hot to the touch; in most instances the traps were checked before 9:00 A. M., several hours before the sun caused maximum heating. Such individuals may have licked the fur of their chests in an attempt to lower their body temperatures. Although mice characteristically salivate before succumbing from heat, these individuals had moist fur over the entire chest and upper parts of the front legs, indicating licking. Mice killed by exposure to heat or cold usually were juveniles or young; subadult and adult individuals of both species were more tolerant. Older animals would be expected to have better homeostatic controls than younger individuals.

HABITAT PREFERENCE

In Mesa Verde P. truei and P. maniculatus occur together only at the fringes of the pinyon-juniper woodland, where ecotonal areas provide less than optimum habitats for both species. Almost all individuals of P. truei occur only in pinyon-juniper woodland, whereas P. maniculatus occurs only in more open habitats, such as grassy meadows and stands of sagebrush.

Pinyon mice were abundant in a variety of associations within the pinyon-juniper woodland. The highest population densities were in pinyon-juniper woodland having an understory of mixed shrubs. In such an association, Poa fendleriana usually is the dominant grass in the ground cover. P. truei was especially abundant along brushy slopes where mixed shrubs (Amelanchier, Cercocarpos and Fendlera) were codominant with pinyon pines and Utah junipers. The pinyon-juniper-mixed shrub area west of Far View Ruins was almost optimum habitat for P. truei.

P. truei was abundant on the rocky ridge of Wetherill Mesa near Mug House; the pinyon-juniper woodland here has a Cercocarpos understory, and appears to provide close to optimum conditions for this species.

Not all associations of the pinyon-juniper woodland support large numbers of P. truei. Pinyon-juniper woodland having a ground cover of Poa fendleriana, and no shrubs, supports few mice; the woodland on Wetherill Mesa near Long House is an example. Juniper-pinyon woodland having a Purshia tridentata understory also supports only a few mice. Such areas occur on the southern ends of the mesas and are characterized by widely-spaced trees and little ground cover--a reflection of the relatively low amounts of precipitation received by the southern end of the park.

P. truei was not found in grasslands on Navajo Hill, or in meadows at the southern end of Moccasin Mesa. The old burned areas on the northern end of Wetherill Mesa and on Morfield Ridge now support numerous grasses and shrubs, but P. truei appears not to live there.

P. truei tends to avoid stands of sagebrush, or grasslands, lacking pinyon or juniper trees. P. truei may venture into such areas while feeding. This species is found in thickets of Gambel oak and in areas with an overstory of mixed shrubs only when a living pinyon-juniper canopy is present, or when a woodland adjoins these areas.

Rocky terrain apparently is not a requirement for P. truei, since much of the pinyon-juniper woodland that is free of rocks supports large numbers. Optimum habitat, however, had a rocky floor. In such places, rocks probably are of secondary importance, whereas the shrubs and other plants growing on rocky soils are important for food and cover. Rocks likely provide additional nesting sites, and allow a larger population to live in an area than might otherwise be possible.

In Mesa Verde the deer mouse, P. maniculatus, prefers open areas having dense stands of grasses, or brushy areas adjoining open terrain. This species lives in stands of big sagebrush; in grassy areas having an oak-chaparral or mixed-shrub-overstory; and in grasslands 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 grassy areas near the entrance of the park. The trapping areas in the first three mentioned had heavy growths of grass 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-muttongrass 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 grasslands or sagebrush areas. As mentioned earlier, P. truei and P. maniculatus occur together in ecotonal areas between the forest and grassy or brushy areas. In Mesa Verde the deer mouse inhabits exposed grassy 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.