The Mexican spotted owl was listed as a threatened species in 1993 by the U.S. Fish and Wildlife Service. Patterns of habitat use by spotted owls in northern Arizona contrast sharply with owls using conifer forests further south.
The Mexican spotted owl has been the focus of agency concern in the southwest for nearly two decades because of the owl's apparent dependence on old growth timber. However, in northern Arizona, subpopulations occur in rather unusual landscapes, the arid and rocky canyonlands of the Colorado River.
Recently, the extent and characteristics of Mexican spotted owl breeding habitat in Grand Canyon National Park were examined using a geographic information system (GIS) and conducting preliminary field validation of GIS habitat models. Steep canyon habitat was strongly associated with spotted owls in Grand Canyon National Park, and they identified large areas of unsurveyed habitat. Given the owl’s threatened status and evidence of population declines in the southwest, further surveys are needed to estimate the distribution and abundance of spotted owls and locate conservation areas to support long-term management and recovery.
The large amount of potential habitat in the park, along with its rugged setting, precludes a complete census. However, the status of the population in Grand Canyon, a potentially large source population, is relevant to its conservation. Use and refinement of the predictive model in order to target areas of high probability of owl presence will focus the research effort. Information on the distribution and status of the owl is needed by park managers for long term natural resource planning. Baseline habitat data, linked to population abundance, can be used to assess long-term trends for down-listing the species. It is hoped that this research would provide fine-grained estimates of habitat, generate habitat specific estimates of density (an index to abundance), and provide data to support planning decisions by state and federal land managers.
The primary goal was to conduct systematic field surveys of the predicted suitable breeding habitat within the interior of Grand Canyon National Park. Accordingly, the following project objectives were implemented:
Field survey procedures followed standardized protocols developed by spotted owl inventory and monitoring experts. Along the Colorado River between Soap Creek and National Canyon, tributary side canyons and suitable stretches of the river corridor were identified as targeted study areas for fieldwork during the study year. Replicate study sites were established within side canyons and one survey visit to each site was conducted. At each survey site, calling routes were established that systematically traversed suitable patches of spotted owl habitat identified by the GIS model. Along each calling route, calling stations were placed every 0.5 to 1.0 km. At each calling station, callers imitated spotted owls by producing a variety of standard calls for 15 minutes. All calling points were surveyed once during the field season. In addition, visit to all known spotted owl territories located along the river were completed to assess occupancy and reproductive status.
Habitat variables were measured at each calling station to record habitat composition and structure. The micro-site habitat variables included: ground cover type (e.g. rock, bare ground, litter, dead and down logs, grass, forbs, shrubs, and cactus); tree species composition and individual tree dimensions (height, DBH, condition, canopy cover); basal area of large trees; and several canyon structural features (width, height, number caves and ledges). Slope, aspect, and elevation of the survey points were also recorded.
During field surveys within the interior region of Grand Canyon, surveys for Mexican spotted owls were completed at 37 study sites including 251 distinct calling stations. Mexican spotted owl single adults were detected at eight study sites, and pairs of spotted owls were detected at seven sites. Nesting behavior, including agitated contact calls and territorial defense was observed for all pairs, and two owlets, approximately 50 days old were observed. Overall, the field surveys located fifteen new spotted owl territories in the interior canyon wilderness of Grand Canyon. All territories were located in the upper reaches of large tributary canyons within steep and rugged rocky canyon terrain. Habitat measurements were completed at each site as well as an analysis of habitat associations.
The field surveys also detected western screech owls, flammulated owls, great-horned owls, and pygmy owls within the canyon environments. Great-horned owls were quite common and were located in similar terrain to Mexican spotted owls. Flammulated owls were the most common owls observed on the canyon rims associated with Ponderosa Pine forests.
The success of field surveys support the field techniques used to locate spotted owls in canyonland terrain including GIS-based predictive models, field tests, river-based access to interior wilderness, and backcountry access via foot trails to the most remote regions of the park. No personnel were injured during the project despite countless hours spent in rugged and arid backcountry wilderness using foot trials and river expeditions. The project results indicated that nesting and roosting areas used by spotted owls are located in the heads of steep-walled canyons below the main canyon rims in desert habitat types.
Although potential affects of rim-based management actions to the owl’s habitat is currently unknown, these results suggest that most territories are located below the zone of proposed management activities, including trail construction along the south rim and prescribed fire. Because prescribed fires have been designed to reproduce a natural fire regime, the long-term health of the forests may be restored; however, the effects to suitable habitat for spotted owls may, or may not, be enhanced.
The results support the idea that the owl primarily occupies rugged canyonland terrain below the main canyon rims. However, owls do move up to the rims to respond to imitated spotted owl calls. All known breeding sites in Grand Canyon, and thus the associated nesting cores areas, have so far been located below the canyon rims within steep walled sandstone canyons. Although typically associated with mature forest habitat, numerous breeding Mexican spotted owls were located within arid canyonlands widely scattered across southern Utah and northern Arizona. In these locations, the owl was associated with steep sandstone canyons covered by relatively open Great Basin Desert scrub and Great Basin Conifer Woodland vegetation communities. These canyonland breeding habitats are rather unusual considering the classic late seral forest habitat requirements typically reported for the owl. The results in Grand Canyon, and surrounding canyonland environments, indicate that rocky canyon habitat is a common and important cover type for the owl and the region clearly supports numerous source populations.