Chapter 1: Regulatory Context, Study Methods, and Voluntary Development Guidelines

Mitigation Hierarchy: Avoid, Minimize, Compensate

Updated September 01, 2021

Birds, bats, and other wildlife are present everywhere in the U.S. As such, it is not possible for wind energy facilities to avoid overlap with wildlife. Wind energy developers and wildlife management agencies therefore focus on what can be done to mitigate adverse impacts, which may include collisions as well as habitat impacts. Agencies and developers follow an agreed-upon mitigation hierarchy to first avoid adverse impacts; minimize impacts that cannot be avoided; and offset, or compensate for, unavoidable impacts.

Avoidance

The first and sometimes best opportunities to avoid adverse impacts occur early in the wind energy development process, before construction begins. As the name suggests, site “prospecting” is the initial, exploratory phase in which project developers look on a relatively broad geographic scale for areas that meet threshold criteria for developing a wind energy project. These criteria could include quality of the wind resource, access to energy markets, and potential for impacts to wildlife. In terms of avoiding adverse wildlife impacts, “red flags” may include areas with high conservation value, unique or rare natural communities, major avian migratory routes, or ecological communities (such as wetlands or native prairie) that provide critical habitat for endangered species or species of concern.

Once the focus has narrowed from a broader geographic area to the consideration of specific sites, developers can look more closely at the potential wind project footprint for features that may warrant particular attention or concern or may create risk to a particular species during construction or operation. For example, are there threatened or endangered species that are present either year-round or during specific periods? Are there areas that are used by wildlife for nest sites or stopover habitat?

The answers to these prospecting and screening questions can help project developers decide both whether and how to proceed, as detailed in the U.S. Fish and Wildlife Service Wind Energy Guidelines. Some sites may be deemed unsuitable; others may warrant additional study to better assess the risk of adverse impacts.

Once a site is selected, project design – including the micro-siting of turbines and other project infrastructure – is informed by existing knowledge and on-site field studies to document the relative abundance, behavior, and habitat use of species that may be sensitive to project impacts.

Learn more about Landscape & Site Assessment to Address Risk.

Minimization

While some adverse impacts can be avoided through careful siting, birds and bats are found in every part of the country, and the risk of collisions can never be completely avoided. Additionally, evidence suggests that the presence or absence of bats in an area before a wind project is built may not correlate with the risk of collision fatalities once a project is operational. Researchers continue to study which groups and species are most at risk of colliding with wind turbines, and stakeholders have developed an array of strategies and technologies to minimize the risk of collision for species of concern. The two basic approaches to minimizing collision risk are deterrence and curtailment.

Deterrence strategies actively discourage birds or bats from entering the high-risk rotor-swept zone using visual or auditory signals designed either to help birds or bats perceive and avoid the risk, or simply to repel them from approaching the turbine. Curtailment strategies involve stopping turbine blade rotation when collision risk is high. Better understanding the conditions associated with collision risk will make it possible to further refine curtailment strategies such that both risks to birds and bats, as well as power generation losses, are minimized.

Learn more about Minimizing Collision Risk to Wildlife During Operations.

Compensation

Compensatory mitigation is required for species listed as threatened or endangered under the Endangered Species Act or the Bald and Golden Eagle Protection Act. If it is determined that a wind project may result in fatalities or the loss of habitat needed to sustain these species, the project developer must come up with a plan to compensate for, or offset, any potential losses. Compensation may take various forms, such as the preservation of high-quality habitat, restoration of degraded habitat, funding of conservation programs, or specific actions shown to reduce fatalities to a given species from other causes. Conservation plans may also be implemented on a voluntary basis to preclude a species from becoming endangered.

Learn more about Compensatory Mitigation.