Assessing the relative importance of source distance determined by acoustic cues and noise level as predictors of disturbance in a study of the Mexican spotted owl

Abstract

In forested areas, acoustic localization of source distance and noise level are potentially reliable indicators of an approaching source. The relative importance of these cues was examined during a study of Mexican spotted owl responses to human-made noise. The study was conducted in mixed-conifer forest on steep northeast facing slopes. Owls had no or limited experience with the experimental paradigm. Shaped noise bursts were projected from a Sound Projections portable stereo speaker system with line of sight to the target owl(s). Signals were shaped noise bursts with 10–15-dB/s onset rate projected to produce received maximum A-weighted levels in two ranges, 55–65 dB and 75–85 dB, at distances of 20, 40, and 80 m. Levels were measured a short distance from the target owl(s) using a calibrated recording system (DAT or digital recorder equipped with an ACO 7013 microphone, 20 Hz to 20 kHz). They were also estimated using a single-event sound propagation model (NMSIM,Wyle Laboratories). Preliminary analysis showed that both source level and distance were significantly related to owl response, but that distance explained twice as much variance in behavior as level. [Work supported by ACC/CEVP; USFWS Permit No. TE024429.]