Since the 1940’s, the Snake River Canyon and surrounding shrubsteppe plateau in southwestem Idaho have been recognized as an important nesting area for birds of prey. The area supports one of the highest densities of cliff-nesting raptors in the world (U.S. Dep. Inter. 1979). During the 1970’s, extensive research indicated that raptors nesting in the canyon were dependent on prey inhabiting the plateau (benchlands) adjacent to the canyon. In 1980, the Secretary of Interior created the 195,469-ha Snake River Birds of Prey Area to protect both raptor nesting and foraging habitats. The area received permanent federal protection on 4 August 1993 with the designation of the 196,225-ha Snake River Birds of Prey National Conservation Area (NCA) (U.S. Dep. Inter. 1995).
Military activities in the NCA occur in the Orchard Training Area (OTA), a 56.022-ha tank training site operated by the Idaho Army National Guard (IDARNG) since the early 1950’s. Currently, the area contains stationary ranges from which tanks and other weapons are fired, artillery firing points, an artillery impact area, tank tracking areas, target systems, and communications facilities. Training activities are heaviest during the late spring and summer. In 1986, IDARNG proposed an upgrade of its facilities in the OTA, including the construction of a sophisticated range complex for tank training.
In 1986, the effects of military use of the OTA on raptors, prey, vegetation, and soils were unknown. No studies had been conducted to determine the levels or significance of raptor use of the OTA relative to other portions of the NCA, and there were few studies of the effects of military training on wildlife elsewhere. In addition, wildfires had burned nearly one half of the NCA since 1980 (Kochert and Pellant 1986), destroying a significant portion of the area’s native shrub communities. Burned areas are now dominated by non-native annual plants, including cheatgrass (Bromus tectorum), Russian thistle (Salsola iberica), and tumblemustard (Sisymbrium spp.), all of which senesce each summer and burn readily during the fire season (Yensen 1982). Wildfires are more frequent in areas dominated by these species, and they burn larger areas than under native conditions (Pellant 1990). This has led to landscape-level habitat conversions in the NCA.
In response to these concerns, a joint BLM-IDARNG research effort to assess the effects of habitat alterations on raptors in the NCA began in 1990. Five component studies were developed to address questions related to raptor distributions in the OTA; raptor use of the OTA in relation to military activities; raptor home range sizes; raptor nesting densities and reproductive success; and the habitat relationships, abundance, and reproduction of predominant prey species. Pilot efforts for each of the component studies were conducted in 1990, followed by 4 years of more intensive research.
This thesis was conducted in conjunction with a study designed to assess military and fire effects on raptor nesting densities and reproductive success throughout the NCA (U.S. Dep. Inter. 1991). My emphasis is on raptor nesting surveys north of the Snake River Canyon, conducted between 1991 and 1994, although I include earlier data from canyon-nesting ferruginous hawks in some analyses or fire effects. The thesis contains 3 chapters: Chapter I examines the effects or military activities on nesting benchland raptors; Chapter II evaluates methods used to assess raptor abundance and reproductive success in the benchlands; and Chapter III assesses the effects of fire-induced habitat conversions on ferruginous hawks throughout the NCA.