Modeling spatial variation in density of golden eagle nest sites in the western United States

Abstract

In order to contribute to conservation planning efforts for golden eagles (Aquila chrysaetos) in the western U.S., we developed nest site models using >6,500 nest site locations throughout a >3,483,000 km2 area of the western U.S. We developed models for twelve discrete modeling regions, and estimated relative density of nest sites for each region. Cross-validation showed that, in general, models accurately estimated relative nest site densities within regions and sub-regions. Areas estimated to have the highest densities of breeding golden eagles had from 132-2,660 times greater densities compared to the lowest density areas. Observed nest site densities were very similar to those reported from published studies. Large extents of each modeling region consisted of low predicted nest site density, while a small percentage of each modeling region contained disproportionately high nest site density. For example, we estimated that areas with relative nest density values <0.3 represented from 62.8-97.8% ([Formula: see text] = 82.5%) of each modeling area, and those areas contained from 14.7-30.0% ([Formula: see text] = 22.1%) of the nest sites. In contrast, areas with relative nest density values >0.5 represented from 1.0-12.8% ([Formula: see text] = 6.3%) of modeling areas, and those areas contained from 47.7-66.9% ([Formula: see text] = 57.3%) of the nest sites. Our findings have direct application to: 1) large-scale conservation planning efforts, 2) risk analyses for land-use proposals such as recreational trails or wind power development, and 3) identifying mitigation areas to offset the impacts of human disturbance.

Fig 1. Geographic extent of study area with boundaries of modeling regions (1–12) shown in colored fill and projection regions (13–15) with crosshatch.

Fig 2. Bar graphs of mean (±2SE) predicted and observed number of golden eagle nest sites in each of 10 relative nest density (RND) bins within each modeling region.

Both predicted and observed numbers were from the 25% withheld data from 10 cross-validations of each region’s model.

Fig 3. Scatterplots of predicted versus observed number of golden eagle nest sites among 10 RND bins within each modeling region’s sub-regions.

Numbers in each region’s plot refer to sub-regions. Sub-region names can be found in S2 Fig.

Fig 4. Bar chart of distribution of observed and predicted nests sites (observed–predicted) within RND bins for 553 independent nest sites within nine modeling regions.

Fig 5. Bar plots of the distribution of predicted and observed nest sites among RND bins in the Madrean Archipelago, and Uinta Basin and North Park.

Fig 6. Percent area within ten equal-sized relative nest site density (RND) bins in modeling areas (gray bars) and modeling regions (white bars).

Fig 7. Maps of the distribution of RND in all regions.

Fig 8. Observed densities of golden eagle nest sites used in model training within ten equal-sized relative nest site density (RND) bins, for each of twelve modeling regions.