While preliminary site visits to a prospective wind energy site are predominantly qualitative (evaluating the overall picture of wildlife and natural resources), the next step in site assessment – pre-construction field studies – is designed to document site wildlife and habitat resources in scientifically rigorous, quantitative terms. The methods used to quantify the distribution, relative abundance, behavior, and site use by species of concern will depend on which species of concern have been identified as either being present or likely to use the site on a regular basis.
Measures of abundance and activity
Commonly used methods for estimating the spatial distribution and relative abundance of wildlife depend on the type of species being documented.
- For eagles and other diurnal birds, methods include counts of birds seen or heard at specific survey points (point count), along transects (transect surveys), and observational studies.
- Raptor nest searches identify both the presence of breeding birds and specific locations within a project site where nests occur (many species of hawks and eagles re-use nests for multiple years). Data collected during these searches allows developers to avoid putting turbines and other infrastructure in areas that might disrupt nesting.
- Populations of prairie grouse are generally assessed by lek counts (a count of the maximum number of males attending a mating ground, or lek) conducted during the breeding season.
- Acoustic monitoring is often used to determine the presence of threatened, endangered, or otherwise rare species of bats throughout the stages of a proposed project.
- Searches of roosts (the places bats sleep during the day) and roost exit counts may be used to confirm bat presence, composition, and colony size.
Learn more in the U.S. Fish & Wildlife Service Land Based Wind Energy Guidelines, Chapter 4.
The Service recommends consulting with state, Tribal, and federal agencies regarding specific survey protocols for locally listed species of concern.
Determining how species use the site
Field studies also need to look at whether there are species that use the site only during certain times or seasons, for example, as migration stopover habitat, wintering, or breeding habitat. In addition to quantifying species’ relative abundance, site documentation gathers information about:
- Temporal use (time of day/night, seasonal use)
- Spatial use (flight height, specific breeding areas or nesting sites)
- Behavioral use (foraging, migration, stopover, breeding, etc.)
- Potential for habitat fragmentation impact on habitat-sensitive species
Utilizing site documentation
Merely establishing that a species is present or that a species is using a site prior to construction is not necessarily a predictor of post-construction risk. While there is a correlation between pre-construction raptor (and particularly eagle) abundance and post-construction fatalities, predicting bat fatalities from pre-construction acoustic monitoring data has not been effective. The identification of nesting or roost sites, or other features of the landscape that may increase collision risk for local species of concern, can be useful in designing a wind project to avoid or create a buffer around those features.
For sites that include habitat used by species particularly sensitive to habitat fragmentation, pre-construction field studies seek to identify the degree to which that habitat will be significantly altered by the presence of the proposed wind energy facility. Again, the specific features documented by the field study – location within the project area of lek and nesting sites, existing instances of fragmentation (such as roads) versus intact areas of habitat – can be taken into account in the design of the project to minimize adverse impacts.
Finally, the data gathered during pre-construction field studies are used to determine whether there is a need for ongoing monitoring of fatalities or evidence of displacement or other habitat-related impacts after the wind project is built, and if so, at what level and for what length of time.