Occupancy of habitats by Mexican spotted owls in relation to explosive noise and recreational access at Los Alamos National Laboratory

We examined 15 years of presence/absence data on the Mexican spotted owl (Strix occidentalis lucida) at los alamos national laboratory in seven areas managed as spotted owl habitat and affected by two types of anthropogenic disturbances: human recreation and relatively frequent but brief impulse noises caused by explosives. on the basis of the percent of years each area was occupied (the occupancy rate), the type of disturbance had an apparent effect on habitat occupancy. the rate of occupancy of spotted owl habitat within 2.4 km of firing sites with restricted access was 31% while in habitat >2.4 km from firing sites in which recreational access was allowed it was 7%. these results suggest that the spotted owl’s use of habitat at los alamos is not adversely affected by noise generated during explosives tests but may be adversely affected by recreational access. the los alamos national laboratory is a laboratory for national security operated for the Department of energy’s national nuclear security administration. the steep-walled canyons of the Jemez Mountains within the laboratory’s boundaries provide nesting habitat for the Mexican spotted owl (Strix occidentalis lucida). Under the endangered species act, the U.s. Fish and Wildlife service listed the subspecies as threatened in 1993 (UsFWs 1993) and published a recovery plan for it in 1995 (UsDi 1995). in the Jemez Mountains, spotted owls prefer to nest on cliff faces in canyons (J. a. Johnson and t. h. Johnson unpubl. data). the effects of noise, especially noise alone without an accompanying physical or visual disturbance, on the spotted owl’s habitat selection is not well understood. Many anthropogenic disturbances have been shown to affect birds of prey (Fyfe and olendorff 1976). Delaney et al. (1999) examined the effects of helicopter noise on the Mexican spotted owl and found that the owls did not respond by flushing when helicopters were >105 m away. Johnson and reynolds (2002) examined the effects of the noise of low-flying jet aircraft on the Mexican spotted owl and found that the owl’s responses to low-flying F-16 jets did not exceed responses to natural events. swarthout and steidl (2001) examined behavioral responses of the Mexican spotted owl at varying distances to hikers and concluded that owls altered their behavior in the presence of a hiker at distances of up to 55 m. Further study (swarthout and steidl 2003) showed that high rates of hikers passing quickly near nests may be detrimental to the owls. Part of the laboratory’s operations includes detonation of a variety of explosives at fixed firing sites on the ground. some of the Mexican spotted owl habitat within the laboratory is near active firing sites. Public access to these areas is prohibited, and work-related access is strictly limited. other spotted owl habitat at the laboratory is not affected by explosives, but peMexiCan sPotteD oWls at los alaMos national laBoratory 103 destrian access is allowed within it, and the public as well as the laboratory’s workforce uses it frequently. the purpose of this study was to compare rates of spotted owl occupancy at los alamos national laboratory in areas near firing sites, subject to relatively frequent impulse noise from explosives, to that in areas not affected by testing of explosives but used for recreation.