Bed-Site Selection by Desert Mule Deer in Southern Arizona

We compared bed sites selected by desert mule deer (Odocoileus hemionus eremicus) to nearby random sites to assess bed site features. Thermal cover of bed sites (i.e., vegetation -75 cm high that provided shade for a deer) was highest in summer (X = 43.7% + 4.1 SE) compared to spring (29.7% + 4.4), winter (33.5% ? 3.1), and autumn (39.8% + 4.0); however, selection for bed sites with thermal cover that differed most from that available randomly was highest in spring (X difference between bed and random sites = 14.8% + 5.2) compared to summer (8.3% ? 4.1), winter (6.6% + 3.5), and autumn (8.6% ? 3.5). Thermal cover is likely important as a contribution to thermoregulation, escape cover, protection of fawns, and fawn survival. RESUMEN-Comparamos los echaderos seleccionados por el venado bura (Odocoileus hemionus eremicus) a sitios cercanos al azar para determinar las caracteristicas de los echaderos. La cubierta termal de los echaderos (o sea, vegetaci6n >75 cm de altura que provey6 sombra para el venado) fue mas alta en el verano (X = 43.7% + 4.1 SE), comparado con la primavera (27.9% ? 4.4), invierno (33.5% + 3.1), y otono (39.8% + 4.0). Sin embargo, la selecci6n de echaderos con cubierta termal que difiri6 mas que lo disponible al azar fue mas alta en la primavera (Xdiferencia entre echaderos y sitios al azar = 14.8% + 5.2) comparada con el verano (8.3% ? 4.1), invierno (6.6% ? 3.5), y otono (8.6% + 3.5). La cubierta termal es quizas importante como una contribuci6n a la termorregulaci6n, cubierta de escape, protecci6n de ciervos, y supervivencia de ciervos. The scale at which biologists measure and assess the ways in which wildlife use landscapes influences the patterns of use observed (Kotliar and Wiens, 1990). Habitat use by ungulates, for example, is studied most often at the home range (macrohabitat) scale with radiocollared animals. Movements, forage, and cover selection by desert mule deer (Odocoileus hemionus eremicus) all have been studied at the home-range scale in the southwestern United States (Dickinson and Garner, 1979; Koerth et al., 1985; Ordway and Krausman, 1986; Bellantoni and Krausman, 1990; Albert and Krausman, 1993), but not at the smaller, microhabitat scale. Studies on use of thermal cover by deer in northern climates suggest that dense cover of conifer forest canopies benefits deer most during winter (Cox, 1938; Verme and Ozoga, 1971; Ozoga and Gysel, 1972; Moen, 1976; DelGiudice and Riggs, 1996). Conversely, temperature extremes in southwest deserts occur during summer; therefore, thermal cover (i.e., vegetation -75 cm high that provided shade for a deer) can aid in thermoregulation (Anthony, 1972; Leopold and Krausman, 1987; Hayes and Krausman, 1993). Our objectives were to determine if deer selected (Hall et al., 1997) bed sites with different amounts of thermal cover compared to paired random sites (50-100 m from bed sites) and to understand vegetation characteristics of bed sites. MATERIALS AND METHODS-We studied deer in Avra Valley, Pima Co., Arizona, in an area bounded by the Tucson Mountains to the east and the Roskruge Mountains to the west, 20 km west of Tucson. Elevations ranged between 610 and 1,429 m. Land ownership was patchy with municipal, state, federal, Tohono O'odham Nation, and private landholders (e.g., housing developments and ranches) represented. 46(3):354-357 Tull et al.-Bed-site selection by mule deer TABLE 1-Thermal cover (%) selection for bed versus nearly random sites for desert mule deer in Avra Valley, Arizona, 1996-1997. Bed Random Bed X differF Season n X SE X SE ence (df= 1, n) P Spring 26 29.7 4.37 14.9 3.99 14.75 8.12 0.0086 Summer 49 43.7 4.11 35.3 4.35 8.32 4.16 0.0470 Autumn 38 39.8 4.00 31.2 4.20 8.58 5.96 0.0195 Winter 45 33.5 3.06 26.9 3.06 6.64 3.65 0.0625 Vegetation associations in the area were common to the Sonoran Desert and included: creosote (Larrea tridentata)-bursage (Ambrosia) in undisturbed flats; desert grassland along portions of major washes and associated flood plains; ironwood (Olneya tesota)-canyon ragweed (Ambrosia ambrosioides) in washes; mesquite (Prosopis velutina)-burroweed (Isocoma tenuisecta) in disturbed flats (i.e., abandoned agricultural lands); and palo verde (Cercidium)-mixed cacti on bajadas. Precipitation is bimodal, occurring during the July to September monsoon and December to March winter showers (Reitan and Green, 1968). Mean precipitation was 28.3 cm for Tucson, Arizona, 1996 (National Climatic Data Center, 1996). Average normal seasonal temperatures for 1993 were 24.3, 30.2, 16.6, and 13.6? C for spring (April toJune), summer (July to September), autumn (October to December), and winter (January to March), respectively (National Climatic Data Center, 1993). These seasons were defined based on weather patterns and biology of desert mule deer (Krausman and Etchberger, 1995). We used net-guns to capture and collar desert mule deer (Krausman et al., 1985) in November 1995 and February 1996 to obtain a representative sample of the mule deer in Avra Valley. Each collar was color-coded to assist in visual identification of