Effects of helicopter noise on Mexican spotted owls

Military helicopter training over the Lincoln National Forest (LNF) in sout hcentral New Mexico has been severely limited to protect nesting Mexican sp otted owls (Strix occidentalis lucida). To evaluate nesting and nonnesting spotted owl responses to helicopter noise, we measured flush frequency, flu sh distance, alert behavior, response duration, prey delivery rates, female trips from the nest, and nest attentiveness during manipulated and nonmani pulated periods, 1995-96. Chain saws were included in our manipulations to increase experimental options and to facilitate comparative results. We ana lyzed stimulus events by measuring noise levels as unweighted one-third-oct ave band levels, applying frequency weighting to the resultant spectra, and calculating the sound exposure level for total sound energy (SEL) and the 0.5-sec equivalent maximum energy level (LEQ(max 0.5-sec)) for helicopters, and the 10-sec equivalent average energy level (LEQ(avg. 10-sec)) for chai n saws. An owl-weighting (dBO) curve was estimated to emphasize the middle frequency range where strigiform owls have the highest hearing sensitivity. Manipulated and nonmanipulated nest sites did not differ in reproductive s uccess (P = 0.59) or the number of young fledged (P = 0.12). As stimulus di stance decreased, spotted owl flush frequency increased, regardless of stim ulus type or season. We recorded no spotted owl flushes when noise stimuli were >105 m away. Spotted owls returned to predisturbance behavior within 1 0-15 min after a stimulus event. All adult flushes during the nesting seaso n occurred after juveniles had left the nest. Spotted owl flush rates in re sponse to helicopters did not differ between nonnesting (13.3%) and nesting seasons (13.6%; P = 0.34). Spotted owls did not flush when the SEL noise l evel for helicopters was less than or equal to 102 dBO (92 dBA) and the LEQ level for chain saws was less than or equal to 59 dBO (46 dBA). Chain saws were more disturbing to spotted owls than helicopter flights at comparable distances. Our data indicate a 105-m buffer zone for helicopter overflight s on the LNF would minimize spotted owl flush response and any potential ef fects on nesting activity.