Book Description

The purpose of this pilot study was to record, characterize, and quantify road maintenance activity in Mexican spotted owl (Strix occidentalis lucida) habitat to gauge potential sound level exposure for owls during road maintenance activities. We measured sound levels from three different types of road maintenance equipment (rock crusherlloader, dozerlroller, and grader), from seven distances (30,60, 120, 180,240,320, and 400 m), in two different habitat types (forested and meadow sites) on the Lincoln National Forest, New Mexico, on 22-23 October 2002 to determine how sound varied over distance, habitat type, topography, and stimulus type. Sound levels increased as the distance between road maintenance activity and microphone locations decreased, regardless of stimulus type or habitat type. Concomitantly, the amount of sound energy within the middle frequency range decreased substantially with increasing stimulus distance from microphone locations. The frequency range over which owls can potentially hear road maintenance events decreased with increasing stimulus distance. Sound recordings of road maintenance equipment were louder at tree microphones than at base microphones, regardless of stimulus distance, stimulus type, and site location. The difference in sound levels between tree and base microphones at each distance was consistently louder at meadow sites (tree microphones were located in trees along the edge of the meadow during testing at the meadow site) compared with forested sites, regardless of stimulus type or stimulus distance. Tree microphones registered a greater proportion of sound energy from road maintenance activities in the middle frequency range than at base microphones, regardless of stimulus type, stimulus distance or site location. Sound level and frequency spectra varied by stimulus type. Rock crushing equipment registered the highest sound levels of any of the road maintenance equipment tested, regardless of stimulus distance or habitat type. Rock crushing equipment had the greatest amount of sound energy in the middle frequency range of all the road maintenance equipment tested, followed by the dozerlroller and the grader, regardless of stimulus distance or habitat type. Road maintenance equipment was consistently louder than background ambient forest and meadow sound levels over a range of distances from 30 to 400 m. The extended duration of both rock crusher sound and the multiple passes required of the grader and dozerlroller are additional, potentially negative considerations. Based on our previous sound research with this species, it appears that spotted owls are capable of hearing all the sound sources tested during this pilot study out to distances of at least 400 m.