PREY ECOLOGY OF MEXICAN SPOTTED OWLS IN PINE–OAK FORESTS OF NORTHERN ARIZONA

We studied Mexican spotted owl (Strix occidentalis lucida) diets and the relative abundance and habitat associations of major prey species in a ponderosa pine (Pinus ponderosa)–Gambel oak (Quercus gambelii) forest in north-central Arizona, USA, from 1990 to 1993. The owl's diet was comprised of 94% mammals by biomass and consisted of primarily the deer mouse (Peromyscus maniculatus), brush mouse (P. boylii), Mexican woodrat (Neotoma mexicana), and Mexican vole (Microtus mexicanus). Spotted owl prey in our study area were smaller on average than prey in other locations, and the total biomass of potential prey was less than that reported in other areas within the owl's geographic range. Although all prey populations exhibited seasonal fluctuations in relative abundance, only the deer mouse exhibited significant temporal variation in population abundance. The general pattern was for prey populations to rise during spring, peak during summer, decline in fall, and reach a winter low. Deer mice exhibited the greatest amplitude in population change as evidenced by the shift from a high of 12.2 mice/ha (SE = 2.3) during summer 1991 to a low of 3.3 mice/ha (SE = 0.7) during winter 1991–1992. Woodrats and brush mice used areas on slopes >20°with relatively more rocks and shrub cover than found in other areas. In contrast, deer mice were found in forests with relatively open understories and little Gambel oak. Conservation measures for the Mexican spotted owl must include management directed at sustaining or increasing prey numbers rather than assuming that managing for owl nesting and roosting habitat will provide favorable conditions for the prey as well. Management practices that increase and sustain shrub and herbaceous vegetation should receive the highest priority. This can be accomplished by thinning small diameter trees, using prescribed fire, and managing grazing pressures.