Eagle Nest Surveys – Insights Gained for Bald Eagles and Implications for the Future

The breeding population of bald eagles in the United States has undergone a remarkable recovery over the past 40 years. Tetra Tech has conducted aerial and ground-based raptor nest surveys throughout the Western U.S in... [ view full abstract ]

The breeding population of bald eagles in the United States has undergone a remarkable recovery over the past 40 years. Tetra Tech has conducted aerial and ground-based raptor nest surveys throughout the Western U.S in association with development of wind energy facilities. Data collected from these surveys have provided a unique and broad-level perspective into bald eagle nesting biology. Here we summarize the insights gained over the course of these surveys and present potential implications for future wind energy project siting and permitting.

We reviewed results of raptor nest surveys conducted at approximately 25 energy projects across 16 states west of the Mississippi River including Arizona, California, Colorado, Idaho, Kansas, Minnesota, Nevada, Nebraska, New Mexico, North Dakota, Oklahoma, Oregon, South Dakota, Texas, Washington and Wyoming. Our variables of interest included proximity to anthropogenic sources of disturbance and proximity to traditional prey sources (e.g., large water bodies). We used data from aerial raptor nest surveys as well as incorporated observations from ground-based nest surveys.

Two broad-scale patterns emerged from these survey results. First, a large number of previously undetected nests were located which is consistent with published studies on increasing bald eagle populations. Second, bald eagles appear to be utilizing increasingly marginal habitats for nesting; we suggest this trend may be a result of optimal habitats becoming saturated. Instances of marginal habitat use included 1) nesting at increased distances from large waterbodies and away from riparian corridors, and 2) nesting in closer proximity to anthropogenic sources of disturbance (e.g., roads, residential areas and wind turbines).

These apparent patterns have several management implications. If survey methods incorporate a habitat-based approach, an increased level of survey effort, including coverage of a broader spectrum of habitat, may be necessary to complete the inventory of all bald eagle nests within a given survey area. Relative to project planning and design, greater numbers of bald eagle breeding territories across the landscape increases the likelihood that a bald eagle nest will occur near a given wind project. Nests located near proposed wind energy projects are considered at risk of disturbance, which has permitting implications. The emerging pattern of nests in proximity to anthropogenic sources of disturbance, however, suggests that the relative risk of nest disturbance may be lower than previously thought. Taken together, these findings suggest that standard survey approaches and current regulatory protections for nesting bald eagles may need to be revisited to adapt to increasing populations and changes in nesting habitat utilized by this eagle species.