Owl Predation on Desert Rodents Which Differ in Morphology and Behavior

Many characteristics of desert rodents are beneficial in avoiding predation. For example, hyperinflated auditory bullae found in the genera Dipodomys and Microdipodops improve auditory acuity and allow the detection of approaching predators (Webster, 1962; Webster and Webster, 1971); elongated hind legs and bipedal locomotion aid in escape (Bartholomew and Caswell, 1951; Eisenberg, 1963). In addition, microhabitat utilization in Dipodomys shifts in response to increased predatory risk due to increased illumination (e.g., Kotler, 1984) or the presence of predators (J. S. Brown, unpubl. data), and even species coexistence may be influenced by predation (Kotler, 1984; Thompson, 1982). However, species lacking in conspicuous anti-predator morphology (e.g., Perognathus, Peromyscus) restrict their foraging to the safety of cover (e.g., Kotler, 1984). Furthermore, Kotler (1984) has suggested that predation has had great influence on community organization of desert rodents through its effect on microhabitat utilization. If this is true, the relative rates of predation on coexisting desert rodent species should be influenced by particular morphological adaptations of rodents, such as bullar volume or types of locomotion (Clarke, 1983; Webster and Strother, 1972), and behavior (microhabitat preference). Animals with anti-predator morphology should suffer lower rates of predation despite using open microhabitats more than do animals lacking such morphology. To assess this, I examined predation by long-eared owls (Asio otus) on a community of desert rodents whose species differ in abundance, morphology, and use of microhabitat while foraging. The study was conducted on a 6 km2 area of semi-stabilized sand dunes in the Great Basin Desert located at Tonopah Junction, 12 km south of Mina, Mineral Co., Nevada (elevation 1,343 m). The dominant vegetation on the dunes is greasewood (Sarcobatus vermiculatus), a stiff, spiny shrub which covers approximately 30% of available space. The rodent species present on or near the dunes included: Merriam's kangaroo rat (Dipodomys merriami), Ord's kangaroo rat (D. ordii), Great Basin kangaroo rat (D. microps), desert kangaroo rat (D. deserti), pallid kangaroo mouse (Microdipodops pallidus), little pocket mouse (Perognathus longimembris), Great Basin pocket mouse (P. parvus), Southern grasshopper mouse (Onychomys torridus), desert woodrat (Neotoma lepida), white-footed deer mouse (Peromyscus maniculatus), and Western harvest mouse (Reithrodontomys megalotis). Dipodomys and Microdipodops are bipedal and have hyperinflated auditory bullae. Perognathus is quadrupedal and has only modestly inflated bullae. The species in the remaining genera are all quadrupedal and lack inflated bullae. Owl pellets were collected from a permanent natural roost of long-eared owls located in the middle of the study site. All pellets were collected prior to the start of the study and then in July and November, 1979; March, July, and November, 1980; and May and July of 1981. All pellets in the roost were collected each time. Pellets were gently picked apart, and all large bones and fragments were removed and saved. Individual prey items were identified to species from entire skulls or large skull fragments. This yielded a count of the minimum number of each species appearing in the pellets each period (Pearson and Pearson, 1947). Rodent populations were censused prior to each pellet collection (except July 1981) by live trapping using parallel census lines and assessment lines (O'Farrell et al., 1977). Assessment line techniques yield a more accurate estimate of the area being sampled by the trap lines, but do not allow error bounds to be placed on the density estimates. Two parallel census lines were spaced 40 m apart; each consisted of 23 stations with stations 20 m apart. At each station, one live trap was placed in each of four microhabitats (Brown and Lieberman, 1973): I. under a bush; II. at the edge of a bush; III. 1 m from the edge of a bush; IV. 2 to 8 m from the edge of a bush. Three assessment lines, 120 m apart, cut the census lines at 45? angles. Each line contained 15 trap stations at 20 m intervals, with 4 traps placed at each station, as described above. During each census, the census lines were trapped first for 2 to 4 nights, until new animals comprised 824 Vol. 66, No. 4