THE EFFECT OF BARN OWLS (TYTO-ALBA) ON THE ACTIVITY AND MICROHABITAT SELECTION OF GERBILLUS-ALLENBYI AND GERBILLUS-PYRAMIDUM

Predation plays an important role ill ecological communities by affect ing prey behavior such as foraging and by physical removal of individu al prey. In regard to foraging, animals such as desert rodents often b alance conflicting demands for food and safety. This has been studied in the field by indirectly manipulating predatory risk through the alt eration of cues associated with increased risk such as cover or illumi nation. It has also been studied by directly manipulating the presence of predators in aviaries. Here, we report on experiments in which we directly manipulated actual predatory risk to desert rodents in the fi eld. We conducted a series of experiments in the field using a trained barn owl (Tyto alba) to investigate how two species of coexisting ger bils (Gerbillus allenbyi and G. pyramidum) respond to various cues of predatory risk in their natural environment. The gerbils responded to risk of predation, in tile form of owl flights and owl hunger calls, b y reducing their activity in the risky plot relative to the control pl ot. The strongest response was to owl flights and the weakest to recor ded hunger calls of owls. Furthermore, when risk of predation was rela tively high, as in the case with barn owl flights, both gerbil species mostly limited their activity to the safer bush microhabitat. The res ponse of the gerbils to risk of predation disappeared very quickly fol lowing removal of the treatment, and the gerbils returned to normal le vels of activity within the same night. The gerbils did not respond to experimental cues (alarm clock), the presence of the investigators, t he presence of a quiet owl, and recorded ''white noise''. Using traine d barn owls, we were able to effectively manipulate actual risk of pre dation to gerbils in natural habitats and to quantify how gerbils alte r their behavior in order to balance conflicting demands of food and s afety. The method allows assessment of aspects of behavior, population interactions, and community characteristics involving predation in na tural habitats.