Common Raven Abundance

Common Raven (Corvus corax) Abundance in Relation to Anthropogenic Resources within the Idaho National Laboratory Site in 2009

     Common raven (Corvus corax) populations in the western United States have increased substantially during the last 50 years. Ravens typically are more abundant in human-altered landscapes than in intact ecosystems. Fragmentation of native habitat is likely responsible for the increase in raven populations by providing an overabundance of anthropogenic resources, such as food, water, artificial perches, and shelter. Food and structure subsidies facilitate raven population growth through increased nest success and recruitment. This is cause for concern among land managers because ravens are considered a synanthropic predator (a predator benefiting from anthropogenic resources and land actions) of numerous sensitive species including greater sage-grouse (Centrocercus urophasianus) eggs and chicks.
     The relationship between land management practices and sage-grouse nest predation is not fully understood. Recent studies have shown that ravens are the primary nest depredators of greater sage-grouse (Coates 2007; Coates et al. 2008). Field surveys of greater sage-grouse nest success show that ravens are a major predator of greater sage-grouse nests occurring on the INL Site and adjacent lands. Concurrent studies based on breeding bird surveys on the INL Site show an exponential growth of raven abundance on the INL Site within the last 20 years (Whiting et al., in preparation). Increases in linear infrastructures (roads and power lines) on the INL Site are likely to further increase raven abundances and subsequently increase greater sage-grouse nest depredation events by ravens. This has the potential to further negatively impact the persistence of greater sage-grouse numbers and population trajectories of greater sage-grouse on the INL Site.     The potential for ravens to limit sage-grouse populations constitutes a need to assess the presence of ravens over large spatial scales within sagebrush steppe habitat.
      This study examines how proximity to anthropogenic resources affects common raven abundance. Determining how anthropogenic resources influence raven density in sagebrush steppe habitat will provide Bureau of Land Management (BLM) land managers with information necessary to identify sources of human disturbance. Information gained from this study will aide in the identification of patterns of synanthropic predation and estimates of their impacts on sage-grouse populations.


Objectives

  • Estimate raven and raptor densities on the INL Site

  • Develop predictive model of broad-scale raven and raptor habitat use

  • Identify anthropogenic factors that affect raven densities

  • Determine the relationship between raven density and apparent sage-grouse nest success.

 

Accomplishments through 2009

  • 334 raven and raptor point count surveys were conducted during the 2009 field season

  • 40 raven nests and 56 raptor nests were identified in 2009

  • Digital geospatial data files were compiled, updated, and incorporated into a Geographic Information System for land use, and anthropogenic subsidies such as water guzzlers used for grazing, roads, facilities, power lines, artificial perches and nesting platforms, radio towers, and landfills on the INL Site, surrounding BLM lands, and local communities.

 

Results
During the 2009 survey season, 176 raven and 157 raptor observations (≥ 1 bird) were recorded. Of the raven nests identified, 68 percent were located on artificial substrate and 32 percent on natural substrate. Of the raptor nests identified, 14 percent were located on artificial substrate and 86 percent on natural substrate.

 

Plans for Continuation

  • Geo-spatial statistical analysis of these data will be performed to determine raven and raptor density in relation to habitat types, distances to anthropogenic resources, and land management activities.

  • Distance sampling techniques will be used to produce a detection function model to obtain estimates of raven and raptor density on the INL Site (Buckland et al. 2001).

  • Habitat selection by ravens and raptors at a landscape scale will be determined by calculating resource selection functions (RSFs) to determine availability versus use.

  • Nesting habitat selection by ravens and raptors at a landscape scale will be determined by calculating RSFs to determine availability versus use.

  • Preliminary analysis shows that ravens are occurring in higher numbers in proximity to linear anthropogenic structures, roads, and power lines. Reasons for these spatial behaviors of raven presence on the INL Site are currently under investigation through further analysis of the data collected.

  • The complete results of this study will be completed by fall 2011, at which time, a more extensive analysis and discussion on factors influencing raven presence will be provided in the form of an M.S. thesis, including raven associations with all habitat cover types occurring on the INL Site.